When air was taken from the front of Dr. Smith's laboratory and air from behind it, near an ashpit, an important difference was found, which maintained itself steadily in one direction through the thirty repetitions of the analysis. The air from the front yielded 20.94° of oxygen against 20-70° in the air from behind. In the latter carbonic acid was also in excess, but not sufficiently so to fill the place of the deficient oxygen, "leading us to look for other gases also that tend to increase the impurity." This he thinks important, since it is certain that gaseous emanations from foul places do not consist solely of carbonic acid. "We see putrid matter laid on the ground, and find it disappearing rapidly, and yet we are told that it is not accompanied by loss of oxygen; it is not credible, and the results given show it to be incorrect." The diminution of oxygen is thus accounted for, but we know that the putrid matter is not altogether resolved into carbonic acid, but partly into other and more deadly gases.

So far as London is concerned, there is less oxygen in the east, than there is in the west end in open places. Hyde Park shows the best results. Numerous analyses were made of air from Scotland, and they show that the seashore and open heaths contain the highest amount of oxygen; but it would seem that the air of the Scotch hills is not, as of hills usually, deficient in oxygen; perhaps, as Dr. Smith supposes, because they are not high enough, and also because they draw their supplies from the sea, which is never far away from them on any side. Scotch readers may possibly feel that Dr. Smith speaks somewhat irreverently of their mountains when he calls them hills and accuses them of want of altitude; but, we suppose that he had to take them for what they are, and that, in a scientific inquiry, fancy was forbidden to add even one cubit to their stature.

After all, this chemistry of climate is no new thing, for in 1828 De Saussure wrote "The quantity of carbonic acid in the open air in the same place is subject to almost continual change, equally with the temperature, the winds, the rain, and the atmospheric pressure," and he even concluded that at Chambeisy the winter months of December, January, and February were as clearly distinguished from the summer months of June, July, and August by the proportion of the carbonic acid in the air, as they are by temperature, rainfall, or windsthe quantity of carbonic acid in the first three months being to

that in the second as 77 to 100. Experiments have not been made to establish or refute this remarkable statement. De Saussure's observation, however, to the effect that the percentage of carbonic acid in the air of mountains is greater than in that of plains has been amply confirmed; while, as we have already stated, it has been also shown that this excess of carbonic acid is attended with a diminution of oxygen. This leads to the remark that we appear to have no certain knowledge that a small excess of carbonic acid is of itself productive of much evil. In mountain air, indeed, it might be regarded as an evidence of purity, if its excess were due to a full combustion of all organic matters in the air. But as a general rule, any excess of carbonic acid should be accepted as a sign and proof of impurity, if for no other reason, because of the bad company it generally keeps. Sulphuretted hydrogen, marsh gas, hydrogen, and a host of other things appear with it as products of decomposition or putrefaction, and many of these are very deleterious. Its presence, too, betokens an active consumption of the oxygen, and a consequent diminution of its quantity. The atmosphere of a room is rendered unpleasant by 1 per cent. of carbonic acid, and fair ventilation means air with less than 06. Yet we can scarcely believe that the unpleasantness and unwholesomeness of an atmosphere with 1 per cent. of CO, is due to the CO,, for Schlagentweit found 09 in the air of the Himalayan mountains, which the senses pronounced pleasant and invigorating.

One cannot help wondering that the air of large manufacturing towns contains so trifling an excess of carbonic acid; but Dr. Smith shows by calculation that in such a town as Manchester, which consumes 2,000,000 tons of coals annually, and which has half a million of breathing people, the increase of carbonic acid in the air should not be more than seen in the second decimal place, and experiment confirms the calculation. Differences, however, which are seen only in the second decimal place, are by no means insignificant or contemptible. In the case of some gases, as, for instance, chlorine, sulphuretted hydrogen, or sulphuric acid, quantities which might thus be represented would render the atmosphere intolerable. Indeed, these small amounts cannot be utterly insignificant, since there is no practical difficulty in measuring them.

The section of Dr. Smith's work, which treats of the air of mines is extremely interesting, and it is with unqualified horror that we read of the extremely degraded or impure airs in which some miners work. A candle will not burn with less than 18 per cent. of oxygen, when 3 per cent. of carbonic acid is present, but men occasionally work where candles will not burn,

and often work where candles burn feebly. "If we leapt," says the author, "from the pure air into a close end we should recoil with horror," but usually the miner passes gradually from the pure to less pure air, acquiring a toleration of the bad as he goes on, till he reaches an atmosphere in which all kinds of impurities are present, the result of exploded gunpowder, burning candles, expired air, mechanical operations, &c. In the experiments made in a close chamber, the air was rendered so impure that the candles and a spirit lamp carried by those who entered it were soon extinguished, and it was found impossible to rekindle them with matches-still gas burned brightly, and human beings breathed without difficulty at first, though discomfort and distress were soon felt. After the air became so impure as to extinguish the gas, and to extinguish candles as quickly as if they had been plunged into water-the oxygen being reduced to about 17 per cent.-still human beings entering the chamber were able to breathe without any immediate distress. In these cases the impurity consisted chiefly of CO2, and to the senses there was less unpleasantness than is often felt in a crowded schoolroom, in the air of which organic impurities also abound. The senses, indeed, though very delicate and acute in detecting impurities, cannot always be trusted, and are clearly quite unable to measure degrees of closeness-01 per cent. of CO, being sometimes more disagreeable to the senses than 4.0, though the latter amount has rapidly caused alarming symptoms. In detecting the mere presence of certain substances, however, the nose appears to be greatly more delicate than chemical reagents. For instance, "one sniff" (4 cubic inches) of air containing only 003 per cent. of muriatic acid reveals the presence and nature of the impurity, but we should have to operate on 60 cubic inches at least with a silver solution before we could reach the same conclusion.

"It has been a question," says Dr. Smith, "if CO2 really does any active harm, or if it is only negative to life. I have always considered it remarkably innocent in small quantities, and been accustomed to look on the organic substances as the real evils; but the experiments in the close chamber have so far changed my mind that I am inclined to think carbonic acid has an (actively) injurious influence."

This is a point which is difficult of proof, but it appears to be supported by such a fact as that air which extinguishes a candle may cease to do so when the CO, is washed out. Doing this, however, would change the percentage of oxygen, and thus, perhaps, explain the phenomenon. Indeed, we know that 2 per cent. of CO, in air, when the oxygen of the

air has gone to form it, will extinguish a candle, while 4 per cent. at least is needed, when the acid, elsewhere formed, is poured into the air. The smaller amount of CO, in the first case involves a much greater reduction of the oxygen, than the greater amount does in the second, and the extinction of the candle is not due solely to the excess of carbonic acid, but also, and perhaps chiefly, to the defect of oxygen.

Dry air, the author tells us, stimulates the skin, because it removes moisture, and the skin must set to work to renew it. Its first action, therefore, should be cheering or bracing, and its last, perhaps, irritating. Moist air again, or soft air, as it is called, is soothing-it calms both the mind and the body, as it does the burning of a candle or a fire. There is a quaintness in this association of the effect of warm moist air on body and and mind with its effect on candles, and Dr. Smith's meaning will, perhaps, be better understood if we let him speak in confirmation of the popular belief that fires burn most brightly in cold dry weather:

"The warmth," he says, "cannot do otherwise than assist the combustion; we learn this on the grand scale by the effects of the hot blast. In cold weather a fire burns brightly, even though the cold is an agent of an obstructive kind to the combustion. In warm weather the fire burns less clearly, though the heat is an assistance to combustion. The reason seems to lie in this fact, that the cold air is deprived in a great measure of its moisture. Its effects on the candle are such as the least observant eye may remark, and the changes on a fire are of the same kind."

In this notice it would be impossible to give the scientific explanation of these phenomena, which the author attempts.

There is another popular belief, to which also he gives his support the notion, to wit, that ventilation is more needed in summer than in winter, and what he says on this point seems worth the quoting.

"I was led," he observes, "to consider one of the effects of heat when working on the gases of putrefaction. It was then perfectly clear that the putrefaction proceeded exactly as the temperature rose, not ceasing at a little above 130°, perhaps approaching nearly 140°—which marks another point in organic substances of importance, as being that at which albumen coagulates. At any rate, according as the temperature rises within any limits natural to this country, the necessity for ventilation is felt. The organic matter becomes especially annoying, even with a small increase of carbonic acid. The substances that annoy us are often volatile, and we may suppose that they begin to decompose in the air when the temperature rises. Indeed it is scarcely a supposition; it is a fact that after a time they become entirely changed, whether they be organic or inorganic. Sulphuretted hydrogen becomes oxidized,

and the unpleasant odours in mining galleries become the smeli of apples. In a house, the organic odours, if shut up, become mouldy or putrefactive, according to circumstances; but if the last they are more rapidly remedied by abundance of air, which oxidizes and washes them away. At a low temperature they will lie unaltered on every substance, and start out again when they are warmed. This action is more distinctly seen when the quantities of oxidizing matter are large; then the eye can perceive the amount daily diminishing until it is all carried away.

"If this result be caused by the action of warmth it is not right to demand as much air for ventilation in a cold day as in a warm; it is not right to break the windows of patients in winter, and to tell them that fresh air is better than heat. The chemical action, and with it the feelings, demand warmth first above all things. It is the very first demand as no function can go on without it. In the railway carriage as well as in the house, the great instinct of man is first to be warm enough, and he is quite right."

There is surely both error and truth in all this. Of course we are injured by low temperatures as well as by foul air. But why be injured by either? Can we not get warmth in pure air? It is true, as he says, "a draught of cold kills like a sword," while, as a rule, foul air kills slowly and insidiously; but we need not strip the last of its bad character, because we choose sometimes to put up with it for a time, in order to escape injury from the first.

We cannot part company with this quotation without pointing to it as a specimen of the author's peculiar and perplexing style, which often renders it difficult to reach his full meaning, and which would look like carelessness, if it were not manifestly what may be called a characteristic. Perhaps we may accept the general plan and arrangement of the book as an outcome of the same characteristic, for between the book as a whole, and many of the chapters, there is, in this respect, a striking resemblance. The author's reputation, however, rests solidly on the good work he has done, and not on his skill as a bookmaker. At page 231, in justification of his mode of expounding his subject, and of communicating information, he frankly says "It does also save a good deal of trouble in compiling "-but trouble so saved to the writer is trouble given to the reader; and it would certainly have been well if Dr. Smith had spared himself as little in the arranging as he has done in the collecting of his facts and information.

For the interesting and instructive results of the numerous examinations of rain collected in localities nearly as numerous, and for the conclusions which may be drawn from these as to the impurities in the air through which the rain has fallen, the work itself must be consulted, as the space at our command has been