on the Atomic Theory, 119 



Hence it is obvious that the system of atomic weights derives 

 no immunity from error, by calling oxygen 1, and hydrogen 

 0.125. The following manifesto of Dr. Thomson is therefore 

 founded on a false conception ; " whereas, if we make choice of 

 oxygen for our unity, any error respecting the atom of hydrogen 

 will be confined to that atom, and will not affect the accuracy of 

 the atomic weights of other bodies*." On the contrary, an 

 error respecting the hydrogen atom will affect the atomic num- 

 bers of all the hydrogen acids and hydrates, under which title 

 rank most of the free acids, a great proportion of the salts, and 

 all the organic products. 



The second reason assigned by Dr. Thomson, that hydrogen 

 combines with but few of the other simple bodies, is a very sin- 

 gular one ; for hydrogen forms a variety of most interesting 

 compounds with all the simple bodies non-metallic, as also with 

 some metals, while its compound, water, is of almost universal 

 agency. 



Three advantages accrue from the assumption of hydrogen, as 

 the atomic unity. 1 . We get rid of numbers less than unity in 

 the scale of equivalents. 2. We avoid fractional quantities 

 throughout the whole range of atomic numbers. 3. The atomic 

 numbers in the hydrogen scheme, exhibit for the most part in 

 reference to that gas, the specific gravities of the chemical 

 bodies supposed to be in the aeriform state ; and the combining 

 ratios of their weights, under the same volume, are made manifest 

 to the eye. This is a capital advantage in all researches on the 

 gases, or on bodies which afford gaseous products in analysis. 

 Let us consider this proposition, in reference to a few leading 

 chemical bodies, Carbon, Phosphorus, Azote, and Sulphur. 



1. Subcarburetted hydrogen gas is, as its name indicates, a 

 compound of one volume vapour of carbon + 2 volumes of 

 hydrogen gas ; the weight of which are 6 + 2 = 8; which 

 sum, from the 3 volumes being condensed into 1, is its specific 

 gravity, compared to hydrogen = 1 . 



2. Carburetted hydrogen or olefiant gas. A volume of this 

 compound consists of 2 volumes hydrogen + 2 volumes carbon, 

 whose weights are § -f- 13 =14; and as these 4 volumes be- 

 come 1 volume of the compound gas, the specific gravity of this 

 will be 14, if hydrogen is 1. 



3. Phosphuretted hydrogen results from 1 volume of phos- 

 phorus as 12 + 1 of hydrogen = 1, constituting 1 volume, 

 whose weight is 13, being the specific gravity, as well as atomic 

 weight, of the aeriform compound on the hydrogen scale. 



4. Subphosphuretted hydrogen is composed of 1 of phosphorus 

 = 12 + 2 hydrogen = 2, constituting 1 volume, whose weight 

 is 1 4. This is at once the atom ana the density. 



* Historical Introduction. 



