CHEMISTRY OF THE EIGHTEENTH CENTURY. 345 



systematic appeal to the balance which constitutes the law of modern 

 chemistry. Here, in the middle of the eighteenth century only, do 

 we find chemistry seeking a basis for its principles in quantitative re- 

 lations. This was the first step of the great transformation of the 

 science from its elementary condition of a mass of facts, connected 

 only by vague theories, into that state in which its general principles 

 rest on the incontrovertible basis of definite quantitative relations. 

 Black tells us that he took 120 grains of common magnesia (carbonate), 

 which he calcined in a crucible, so that it lost 70 grains of its weight. 

 The magnesia thus calcined was then dissolved in a sufficient quantity 

 of dilute vitriolic acid (sulphuric acid}, the solution taking place 

 without effervescence. To this liquid was added a hot solution of 

 fixed alkali (carbonate of potash), and the precipitate produced was 

 collected, washed, dried, and weighed. In this state it had recovered, 

 except a mere trifle, the whole of the weight it had lost by calcination, 

 and in every respect it behaved like common magnesia. This experi- 

 ment confirmed him in the idea that the magnesia receives from the 

 alkali used in the precipitation a certain quantity of air. Soon after- 

 wards Black made an experiment in which he first prepared the gas 

 we now call carbonic acid by the process still in use. He put a little 

 alkaline salt (carbonate of potash), or lime (i.e., old lime or mortar which 

 contains carbonate of lime), or magnesia (carbonate), into a flask con- 

 taining a little dilute acid, and closed the flask with a cork through 

 which passed a glass tube bent like a swan's neck, dipping beneath the 

 mouth of an inverted vessel, as in Hales' method (page 342). He soon 

 saw a brisk effervescence, and many bubbles of " elastic aerial matter" 

 (carbonic acid gas) rose up to reach the surface of the liquid within the 

 vessel. " It is not," he remarks, " a momentary vapour which escapes, 

 but a permanent elastic fluid (gas) not condensable by cold." Black 

 called this " elastic fluid" jixed air, because it had been fixed in a solid 

 condition in the magnesia. It soon afterwards received the name of 

 carbonic acid gas, which it has retained to this day. 



In the same year, viz., 1757, Black discovered that carbonic acid 

 gas (as we shall now call it, instead of following Black by calling it 

 fixed air) cannot be breathed by animals, and that it had even a poi- 

 sonous effect on them. Sparrows introduced into an atmosphere of 

 the gas died in ten or eleven seconds, but if their nostrils were stopped 

 with lard before they were placed in the gas, their death took place 

 only at the end of three or four minutes; He declares his opinion 

 that the change which is made by respiration in originally pure air 

 consists chiefly if not entirely in the transformation of a part of the 

 air into carbonic acid, and he describes the now well-known experi- 

 ment of blowing through a tube into lime-water, which becomes turbid 

 by the precipitation of the lime (carbonate). This experiment, which 

 should be familiar to every one, may be repeated with no other ap- 

 paratus than a clean wide-mouthed bottle. Pour a little clear lime- 



