on the Atomic Theory. 117 



densities to that of hydrogen being apparently in simple arith- 

 metical proportion, naturally pointed to the principle so ably de- 

 veloped by Dr. Prout. 



The proportion of the two elements in the composition of water, 

 is by common consent regarded as constituting the ground-work 

 of the atomic scale. Water is known to consist of one volume of 

 hydrogen combined with half a volume of oxygen ; and if this 

 half volume be exactly eight times heavier than the entire volume 

 of hydrogen, we shall have their atomic relation represented by 

 these numbers. This half volume of oxygen, viewed as a com- 

 ponent of concrete bodies, is estimated to weigh 1 ; phosphorus 

 then weighs 1.5, and sulphur 2. But if that half volume of 

 oxygen be contemplated in its gaseous state with reference to one 

 volume of air as 1, or unity, its weight becomes 0.5555 ; and 

 the atomic weights of the other bodies are brought into com- 

 parison with this gaseous standard, by reducing their atomic 

 numbers in the ratio of 1.0000 to 0.5555. Thus phosphorus will 

 become in the primitive combining volume of its vapour, 

 0.833 = 1.5 X 0.5555; and sulphur, 1.1111 s= 2X0.5555. 



In some cases, the concrete aspect of the oxygen atom, or 

 prime equivalent =1, is convenient; in others, the gaseous as- 

 pect z= 0.5555. They are, however, merely different forms of the 

 same proposition ; nor can the arithmetical reduction of the 

 concrete unity to the pneumatic expression, be considered, with 

 Dr. Thomson, as a law of combination. In fact, the assumption 

 that half a volume of oxygen constitutes one atom, versus an 

 entire volume of hydrogen, is altogether arbitrary, and merely a 

 matter of convention among chemists. 



We are acquainted with no body, contemplated in its gaseous 

 state, which unites with less than its own volume of hydrogen. 

 Most bodies, on the contrary, which combine with hydrogen, do 

 so in the same proportion as oxygen does ; that is, they take 

 twice their volume of this inflammable gas. Now, if the densities 

 of hydrogen and oxygen gases are as 1 to 1 6 exactly, then from the 

 known relations of oxygen to other bodies, it will not be difficult 

 to shew, that their atomic weights will be all, very nearly, if not 

 exactly, whole numbers, or multiples of hydrogen = 1. Thus 

 we perceive carbon to be 6, oxygen 8, phosphorus 12, nitrogen 14, 

 sulphur 16, &c. 



Dr. Thomson, in his " Historical Introduction," considers 

 Mr. Dalton's choice of the atom of hydrogen for unity, as un- 

 happy ; asserting, that with the exception of Dr. Henry, of Man- 

 chester, and one or two chemical gentlemen in London, " this 

 method has been rejected by almost all the British chemists, and 

 by all the chemists without exception in Europe and America." 

 The Doctor's reasons are thus stated: " 1. Because the atom of 

 nydrogen is the most difficult of all to determine ; and chemists 



