38 CHEMISTRY. 



concerned, and of the bodies produced by the chemical 



change. This is shown thus : 



Hydrogen 

 + H 



57.1 57.1 



The 39.1 stands for so many grammes, pounds, tons, etc., 

 of potassium, the 18 (obtained by adding the weights of the 

 atoms) for so many grammes, pounds, tons, etc., of water; 

 now when chemical action takes place, nothing is lost, so we 

 must find just as many grammes or pounds of these ele- 

 ments taken together, viz., 57.1, on one side of the equation 

 as on the other. That this is the case is evident ; only the 

 atoms are arranged differently on the right-hand side we 

 have 56.1 grammes, pounds, or tons of potassium hydrate, 

 and 1 pound, etc., of hydrogen. 



34. Combination by Volume. "We have hitherto considered only 

 the laws governing combination by weight, but the elementary bodies which 

 exist in a gaseous state combine by volume in simple ratios, and the vol- 

 ume of the resulting body bears a simple ratio to the volume of its constit- 

 uents. A full examination of the laws of combination by volume can not 

 be entered upon in this work. We can give you only a few illustrations to 

 show that the volume relations of gaseous compounds are very simple. 



Thus one volume of H and one volume of Cl unite to form two volumes 

 of hydrochloric acid, two of H and one of O form two of water, three of II 

 and one of N form two of ammonia, and, finally, four of H and one of C 

 form two of marsh gas. This, you will see, is in direct connection with the 

 atomicity of the elements explained in 44. Now there is a law in physical 

 science that all molecules in the gaseous state occupy the same volume ; and 

 taking the volume of an atom of hydrogen as unity, the volume occupied 

 by molecules is two. Chemical formulae, then, dealing with molecules of 

 bodies, as well as the atoms, represent the volumes of the constituents as 

 well as the weights. If we represent the volumes by squares, as on the next 

 page, this will be somewhat clearer ; we take for examples the formation 

 of the bodies HC1, H 2 O, NH 3 , and CII 4 , just mentioned : 



