1878.] " Facility of Production" of Chemical Combinations. 53 



Similarly, tlie " facility of production " of the unit of the hydro- 

 carbon valerylene «V is to the "facility of production" of the hydro- 

 carbon benzene a 3 /c 6 as 126 to 84. The measure of the " facility of 

 production" of a unit of matter being in all cases that number which 

 expresses the different ways in which the operations may be permuted 

 by which that unit is made. It is important to observe that the 

 comparison which is here effected is that between the " facility of 

 production " of the units of homogeneous substances, namely units 

 severally composed of the same number of simple weights. Thus we 

 can compare the " facility of production " of valerylene a 4 /c° and 

 benzene oV, but we have no means of effecting this comparison be- 

 tween the unit of valerylene a 4 * 5 and the unit of propylene a 3 *.- 3 . 



That this " facility of production " is among the causes which de- 

 termine the existence of certain units of matter to the exclusion of 

 others can hardly be denied, as a speculative truth, by anyone who 

 admits that these units are built up from simple weights in the manner 

 I have described, but the action of this cause might be so veiled from 

 our view, by the action of other causes, that we might never be able to 

 detect it by the isolation of its effects. The verification of a scientific 

 theory is effected by comparing the results indicated by that theory 

 with the actual results of experience, and no physical theory can be 

 regarded as demonstrated unless it can stand the test of this com- 

 parison. The system of combinations which in the present instance 

 lends itself most readily to this verification is the system of hydro- 

 carbons. This system contains some eighty-eight combinations, and, 

 although limited as regards the full attainment of the object in view, 

 is yet far more extensive than any other similar binary system. The 

 composition, too, of the several units of the system is well determined. 

 The gaseous density of the hydrocarbon being, in numerous cases, 

 ascertained by experiment, and, where this is not the case, being 

 determined by reasoning, generally of a very conclusive character.* 



Before proceeding further, I will give a list of the hydrocarbons to 

 be considered, and their symbols. For facility of reference, they are 

 arranged according to the powers of a. 



1 unit of space. a 2 * 3 unit of allylene. 



x „ carbon. <x 2 x 6 „ phenylene. 



a „ hydrogen. a 3 * 2 „ ethane. 



a* 2 „ acetylene. a 3 * 3 „ propylene. 



a"/t methane. a 3 * 4 „ crotonylene. 



a 2 * 2 „ ethylene. a 3 * 5 ,, valylene. 



* In compiling this list, pains have been taken not to introduce non-existing 

 hydrocarbons. It has not appeared necessary, in all cases, to verify the list by 

 reference to the original authorities, but it maybe taken as representing the existing 

 view of the subject as given in the best text-books. In making these references, I 

 have been much indebted to my friend Mr. W. IT. Donkin, who has gone into the 

 question with the greatest care. 



