168 ATTRACTION. 



is a contraction to two thirds. In the formation of sulphuretted hy- 

 drogen, and sulphurous acid, there is also a contraction to one half ; 

 and the same fact is seen in many other cases. 



(aaa.) By knowing the specific gravity of the gases composing a 

 compound gas, and the degree of condensation which they undergo, 

 the specific gravity of the compound gas may be calculated. Dr. 

 Turner has given the following instances among others. Ammonia, as 

 just observed, contains 3 vols. hydrogen, and 1 of nitrogen, condensed 

 into 2 vols. The sp. gr. of hydrogen is 0.0694, air being 1, and that 

 of nitrogen is 0.9 722 therefore the latter number +0.0694X3 = 



=0.2951, the sp. gr. which ammoniacal gas would have, 



4 



were there no contraction of the gases ; but as they contract one half, 

 the sp. gr. will be double of that, or 0.5902, which is its weight, as 

 ascertained by experiment by Sir H. Davy. Nitric oxide gas, be- 

 ing composed of 100 vols. of oxygen, and 100 of nitrogen, united 

 without contraction, must form 200 volumes of the compound, 

 and of course the sp. gr. must be the mean of its components, or 



3LJ _ = 1.0416, which accords with the average results 



of the best experiments. 



(bbb.) The combinations by volume coincide accurately with the 

 law of multiple proportions, for it is obvious that double, triple, fyc. 

 of the volume of a gas must also be double, triple, fyc. of the weight. 

 There is also an additional coincidence, that is not possessed by 

 the compounds that are not aeriform. Although in them there is an 

 arithmetical relation between the weights of the different proportions of 

 the same principle, there is no such correspondence between the dif- 

 ferent principles of the same compound. Thus, between the 14 parts 

 by weight, of nitrogen, and the 8 of oxygen, contained in nitrous 

 oxide ; and the 14 and 16 parts of the same principles, in nitric ox- 

 ide ; and the 6 of carbon, and 8 of oxygen, in carbonic oxide ; and 

 the 6 and 16 of the same principles in carbonic acid, there is no mul- 

 tiple relation. 



(ccc.) In combinations of aeriform bodies, there is a multiple re- 

 lation, not only between the different proportions of the same princi- 

 ple, but of the different principles, that are united in the same com- 

 pound. The table on page 167 proves this proposition to be true. 



(ddd.) In general, a volume of a gas represents a combining pro- 

 portion. Oxygen is the only exception ; in that gas, half a volume 

 represents a combining proportion. This arises from the fact that in 

 the lowest combination of oxygen known, it unites with two volumes 

 of hydrogen, which are supposed to contain only one combining pro- 

 portion, and therefore the combining proportion of oxygen is consid- 

 ered as contained in half a volume of that gas. 



