evaporation from the grass appears to be about 11⁄2 times, and from earth about twice that from water. When the grass is very wet its loss is sometimes 2 times that from water, and that from the soil has been as much as 3 times that from water. This is what might have been anticipated, for the grass greatly increases the surface from which the loss goes on; and the soil, from its irregular surface, not only presents a larger surface than water, but is of a darker colour and absorbs more heat. So far it appears that as the surface dries the evaporation become slower, especially from the soil, the grass being different in so far as it, by means of its roots, pumps up the water from below; it however serves another purpose, viz., to deposit dew, and once the amount of water deposited in this way, during twelve hours, 9 p.m. to 9 a.m., was equal to half the amount of evaporation for the previous twelve hours, and I have no doubt that when we get heavy dews the deposit at night will be sometimes equal to the loss by day, i.e. in Sydney, where the air is very humid; inland such conditions would not often exist.

In order to see how these observations affect irrigation, let us take Albury as a fair sample of the Murray country, and one from which we have eight years of evaporation results measured from water. Taking the months October, November, December, January, February, and March, the mean of eight years gives each month 4.194 inches of evaporation, and if we take only December and January, the mean is 6·892 in.; and as we have seen, in the previous part of this paper, that in the comparatively humid atmosphere of Sydney the grass lost one and a half times, and the earth twice as much as water, we should have at Albury a loss from grass land of at least 6 inches per month if the soil were kept well supplied with water. If however, the supply of water were limited, and the irrigated surface kept comparatively dry, it is probable the evaporation could be kept down to 4 inches per month during (say) January and February, and about 3 during the other warm months, and 1 or 2 during the remainder, in all about 25 inches would be required to irrigate for a whole year. Generally the rainfall in that flat country would average about 20 inches, and when this is fairly distributed it gives abundant grass, so that it confirms the estimated necessary amount given above. It must however, be borne in mind that when irrigation would be most wanted the heat and dryness of the atmosphere would be greater than under the conditions of a good season, and the evaporation would be also greater, and there can be little doubt that provision must be made for a very free use of water at such times.

When I next take up the subject I hope to have a more extended series of observations on which to base conclusions, but I think it is evident that a very large supply of water would be required; and that the estimate is a fair one is I think evident from the following opinion from perhaps the most competent man in England.

At the meeting of the Mechanical Section of the British Association in 1882, John Fowler, Esq., C.E., F.G.S., the President of the Section, stated in his address "that the cultivated lands of Lower Egypt have an area of 3,000,000 acres, and to irrigate this effectually, at least 30,000,000 tons of water per day would be required, an amount somewhat exceeding the whole of the Lower Nile discharge.' This is at the rate of 10 tons of water per day per acre, that is almost exactly a tenth of an inch per day on the surface. The Nile Delta is a flat country, little if any hotter than our interior plains, and situated much nearer to the bably the evaporation is not greater there than with us. tenth of an inch per day at least is 3 inches per month (at least). Of course here every year brings some rain, and irrigation would not be required continuously, but probably for one or two months during every year. Something like 6 inches over the surface would therefore be required at least, and provision should be made for three times that amount.

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Now, as to the quantity of water which the Murray River could supply, some careful observations of the water carried off by the Murray were made by the late P. H. Gell, Esq., for three years, and for three other years I have had daily observations of the height of the river; for its cross section, we have one very carefully made at Ki near Euston, under the direction of the Engineer-inChief for Harbours and Rivers. Taking then these quantities, I find the average (six years) outflow of the river, that is, all the water that passes, floods included, is equal to 4.8 inches of rain over the river basin. A great part of this is flood-water, which passes away when the country is wet and irrigation not required. Of course some of this could be stored by artificial means, but necessarily in large shallow lagoons where evaporation would be very active, so that the river, even if all the water were taken out of it near its source, for irrigation, which is impossible, would only supply a quantity equal to a general rainfall of 3 inches at the utmost. From this, a considerable reduction would have to be made for loss in distribution; by leakage, &c. I am not opposed to irrigation; on the contrary, I think it is the want of the interior of New South Wales, but the one insuperable difficulty that always presents itself to me is to find sufficient water for general irrigation. The rain and river records show a definite quantity; and when I attempt to balance this against the evaporation from the general surface, I always get into the difficulty that I have tried. to present to you to-night, and I have brought it forward in the hope that it will be taken up and fairly discussed in what has yet to be said upon the matter. I can understand that there are no engineering difficulties in the way.

On the discolouration of white bricks made from certain clays in the neighbourhood of Sydney.

By EDWARD H. RENNIE, M.A., D. Sc.

[Read before the Royal Society of N.S.W., 5 September, 1883.]

(Preliminary communication.)

CONSIDERABLE annoyance has of late been caused to builders in Sydney by the extraordinary colours which develop in certain bricks, after exposure for some time to the air.

The bricks which

appear to be particularly liable to this change are those which are moulded under considerable pressure, and afterwards burnt at a comparatively low temperature, in order to prevent the deep colour produced by "hard" burning.

Messrs. Mansfield, being desirous of ascertaining the cause of these appearances, took the matter up, and the investigation came into my hands.

One of the bricks sent by these gentlemen (from the works of Messrs. Monro Goodsell Bros., near Newtown) had been lying in one position for two years. On the side facing the south it was almost entirely covered by a green substance, which, on microscopical examination, proved to be of organic origin, and to consist of one of the algae, probably protococcus. On one of the edges of this same brick, however, a yellowish-red patch was observed. On breaking off some of the coloured portion, and putting it into water, a yellow solution was obtained in which no trace of organized matter could be detected by microscopical examination. At this time my attention was called to the discolouration of the bricks now being used in various buildings in this city. Some of these were procured, and from them coloured solutions were obtained in which no vegetable cells could be found. One of these bricks was exhibited at a meeting of the Microscopical Section of this Society, and neither at that meeting nor on subsequent careful examination by Dr. Morris could any distinct vegetable structure be found.

This led to a chemical investigation of the yellow or green liquid obtained by simply soaking the coloured portions of the bricks in water. It was found that this solution gave all the reactions of vanadic acid.

Since then other bricks from various localities near Sydney have been examined. All show, more or less, the same yellow and green colourations, and these have been proved to be due to vanadic acid.

The presence of vanadium in several clays has been pointed out by Roscoe, and its effect on bricks has been noticed in Germany. Seger has found (Thonindustrie Zeitung, 1877-78) that the goldenyellow and grass green colours which appear in some white bricks from the neighbourhood of Wittemberg are due to vanadic acid. In one case the residue obtained by evaporation of an aqueous solution contained over 44 per cent. of potassium vanadate. Under the article "Vanadium,"in Muspratt's Chemie in Anwendung auf Künste und Gewerbe (3rd edition), it is stated the production of these coloured compounds depends to a large extent, as might Je expected, upon the temperature and nature of the furnace used in burning, and that at a very high temperature an insoluble silicate of vanadium is formed.

Probably the only method of prevention in this case is to use a higher temperature in burning; but then it is no longer possible with ordinary clay to produce such light-coloured bricks.

It will be seen, therefore, that discolourations such as those described above may be due to one or two essentially distinct causes, either

(1) Growth of vegetable matter under favourable conditions, or (2) Formation of coloured metallic compounds, especially metallic vanadates, in the process of burning.

It may be mentioned here that Forbes (Dingler's Journal, 192, 116) found in a red clay from Shropshire, England, besides vanadium, titanium, cerium, chromium, molybdenum, copper, and even gold. As many of these metals form coloured compounds, it is possible that, in some cases, colours may be produced due to the presence of other substances besides vanadium.

Pressure of work of another kind has prevented me from determining the quantity of vanadium in the clay from which these bricks are made, but I hope before long to make analyses of various clays in the neighbourhood of Sydney, with a view of determining this point, and also of inquiring into the presence or absence of molybdic and phosphoric acids. The former has been found in certain clays, and the latter is said to invariably accompany vanadic acid.