signed them for different and distinct purposes of utility. You have enumerated the separate uses of most of the earths-Can you recollect the collective advantages which arise from this class of bodies? The uses of many of these earths are not yet discovered; but the benefits which we derive from lime, clay, silex, and magnesia are very various and important. In order to impress your memory, and to finish the consideration of this class of bodies, endeavour to recollect some of the more important uses of those earths with which you are best acquainted? LIME, then, has an extensive and important use in agriculture; it is employed in building, &c. and adds much both to the neatness and durability of our dwellings. SILEX is the basis of all mortar and cements, and is a necessary ingredient in earthen-ware, porcelain, and glass. BARYTES is employed in chemical laboratories as a re-agent, and for the formation of salts. MAGNESIA, besides being the basis of several salts, is of great use in medicine; and ALUMINE, by a due mixture with silex, is capable of forming vessels for chemists "The goodness of a soil depends upon its being able to retain the quantity of moisture which is proper for the nourishment of vegetables, and no more. Now the retentive power of a soil increases with the proportion of its alumine, lime, or magnesia; and diminishes as the proportion of its silex increases." The Essay on Soils, by Dr. Alderson of Hull, is, perhaps, the best paper that has yet appeared on this subject. that will resist the action of the most concentrated acids; it is the material of which the bricks are formed which construct the walls of our habitations, and is also spread out by the great Author of nature in strata within our hills and mountains,* to arrest the progress of subterraneous waters, and to produce those springs that fertilize the valleys, and which take such diversified courses upon the surface of the globe. * Some have imagined that the earth would have had more beauty, and would have been much more convenient, if the whole had been a plane surface; and have adduced the mountainous parts of the world as a proof of the imperfection of the works of Nature; but surely such persons were not aware that, independent of the beautiful variety they produce, the crystal spring and the meandering river owe their origin entirely to these eminences dispersed over the earth's surface. "I see the rivers in their infant beds! THOMSON. CHAPTER VI. OF ALKALIES. WHAT is the nature of an alkali? The alkaliest have an acrid and urinous taste; they change the blue juices of vegetables‡ to a green, and the yellow to a brown; and have the property of rendering oils miscible with water. They are incombustible, but may be rendered volatile by great heat. They are soluble in water; form various salts by combination with acids;§ * To a person who has not had an opportunity of examining an alkali, no written description that can be given will convey any correct idea of the taste or properties of this class of bodies: let the pupil therefore procure a specimen of each kind, before he enters upon this chapter. The juice of the gooseberry and the lemon, and many other vegetable substances will remind him of the general properties of the acids: but having met with nothing analogous to the alkalies, it will be necessary for him to laste and examine one of these bodies in order to acquire any thing like a just idea of their nature. Let him form potass or soda into a neutral salt by saturating it with one of the acids, and he will perceive still more of the nature of these bodies. See note* page 149. The word alkali is of Arabian origin, and signifies the "dregs of bitterness." Although this effect of alkalies on the blue juices of vege tables is almost universal, we know of one exception. Tincture of litmus, and litmus paper, are always rendered more intensely blue by the addition of alkalies. Potass becomes comparatively mild by its union with carbonic acid; and the most caustic soda, if united to corrosive muriatic acid, forme the mild salt need at our tables. and act as powerful caustics* when applied to the flesh of animals.t How many alkalies are there? There are three alkalies ; two of which have been called fixed alkalies, the other the volatile alkali. Which are the fixed alkalies? The fixed alkalies are potass§ and soda. Why have they been called fixed alkalies? Because they will endure a great heat and still remain unchanged. * It seems that causticity depends on chemical affinity, and that the caustic substance corrodes the matter to which it is applied, in consequence of its tendency to unite with that matter; and that it continues to act upon it until it has saturated itself by the combination. Thus the most caustic alkalies may be combined so as to form insipid salts. If a piece of animal flesh be put into a strong solution of potass or soda, it will immediately be acted upon by the alkali, and soon be entirely dissolved. The alkalies have a great affinity for water and for carbon; it is therefore probable that their causticity is owing to this circumstance, water and carbon being so abundant in animal bodies. If the alkalies be dissolved in a large portion of water, they lose their caustic qualities. The French chemists have classed barytes, strontian, lime, and magnesia among the alkalies, on account of their possessing some alkaline properties; but as they bear a greater resemblance to the earths than the alkalies, I adhere to the old classification of these bodies, particularly, as a new one will probably soon become necessary. Modern chemists call the pure vegetable alkali potass, whereas the article of commerce is called potash. This alkali was formerly procured by burning vegetables in large iron pots; hence it acquired the name of potash. Soda acquired its name from the plant salsola soda, which grows on the Spanish coast, and is burnt for its preparation. Mr. Chenevix objects to the name fixed alkalies; but as the term is used in all modern chemical writings, and as I conceive that it may be useful to assist the memory of beginners, What substances enter into the composition of these alkalies? Till lately the fixed alkalies were considered to be simple substances, chemists not having been able to decompose them; but they are now found to be compound bodies.* Have we any historical account of the discovery of these substances? Potass was known to the ancient Gauls and Germans; and soda was familiar to the Greeks and Hebrews. This latter substance was known 'to these ancients by the name of nitrum.‡ I have retained it in this work. These alkalies have surely some claim to the title of fixed, for they require a red heat to dissipate them; whereas the other alkali becomes volatile at a very low temperature. *It will be recollected, that, in the first edition of the "Chemical Catechism," written seven years ago, I offered this opinion of the compound nature of the alkalies. The late galvanic experiments of Sir Humphrey Davy have confirmed the truth of this conjecture, and proved beyond all doubt that potass and soda are both metallic oxides. See Experiments, No. 248 to 255, and Additional Notes, No. 67. At the great explosion of the iron furnace at Colebrook Dale by the bursting of the dam of the river, on the 7th of September, 1801, its whole contents were thrown into the air, and it is said nothing was found afterwards but potass, soda, and prussiate of potass. It has been ascertained by experiment, that potass is formed in what are termed nitre-beds, or collections of the materials from which nitre is procured, though it could have pre-existed in none of them. How this takes place we certainly at present are quite ignorant. These people were probably the inventors of soap, as we are told by Pliny, that they made soap with the ashes of vegetables and tallow. A soap-boiler's shop with soap in it was discovered in the city of Pompeii, overwhelmed by Vesuvius, A. D. 79. See "Miss Starke's Lellers from Italy.” This substance is found native in Egypt, and is there called |