346 HISTORY OF SCIENCE. 



water into the bottle, cover the mouth of it with the palm of the hand, 

 and shake the bottle vigorously. The lime-water will remain clear. 

 Now breathe into the bottle, quickly cover with the hand, and shake 

 as before. The lime-water will immediately become turbid from the 

 presence of carbonate of lime, formed by the combination of the lime 

 with the carbonic acid of the breath. If the bottle be held for an in- 

 stant mouth downward over a lighted candle, and lime-water be imme- 

 diately introduced and shaken up in it, the effect will be the same as 

 with the breath. 



Black soon found that the air (gas) produced in fermentation is 

 carbonic acid, which in fact had been already recognized in these 

 circumstances by Von Helmont, and described by him under another 

 name (page 230). The same day on which Black made this observation, 

 he proved by means of lime-water that the combustion of coal gives 

 rise to carbonic acid gas as Von Helmont had anticipated. Black's 

 researches establish the fact that the alkalies (pctashes, or carbonate 

 of potash) contain a certain quantity of carbonic acid, which is ex- 

 pelled by the contact of an acid, such as sulphuric acid, but that the 

 strongest heat fails to expel the carbonic acid from its combination 

 with potash. On the other hand, while lime combines with carbonic 

 acid, which^tronger acid also expels, the gas is completely driven off 

 from its calcareous combination by a moderate heat, and the result of 

 the calcination is quicklime, which is caustic, and when exposed to the 

 open air absorbs carbonic acid from it, and thereby loses its causticity. 

 The other caustic alkalies are also in some measure neutralized by 

 absorbing something from the air. Black knew that his "fixed air" 

 (carbonic acid] constituted only a portion of the atmospheric air, and 

 that it is this portion which causes a crust to form on the surface of 

 lime-water exposed to it. 



A Dublin physician, DR. MACBRIDE (1726 1778), developed in 

 some respects the views of Black as regards the part played by carbonic 

 acid in the animal economy, and his Essays had at least the merit of 

 attracting the attention of physiologists to this subject. He pushes the 

 theory of carbonic acid as a consolidating material to an extreme, and 

 even bases his medical practice, or fancies that he does, on these slender 

 theoretical foundations. He declares that all bodies owe their strength 

 and consistence, the cohesion of their parts, to the fixed air they con- 

 tain, and that putrefaction is the result of their losing this fixed air. It 

 is a fact that putrefying bodies disengage carbonic acid gas, but Mac- 

 bride's inference was erroneous when he concluded that this was the 

 cause of their decay, whereas it is in reality one of the effects. He re- 

 commends scorbutic persons to drink liquids containing carbonic acid, 

 because their disease is a putrid malady caused by the want of that prin- 

 ciple which is the bond and cement of the body. On the other hand, 

 clear as were Black's doctrines and convincing his experiments, they 

 were far from receiving immediate assent. Thus Meyer, a German apo- 



