6 Mr. William Sutherland on the 



two terminal hydrogen atoms H + H are apparently repre- 

 sented by an initial term 2*2, which is more than twice as 

 large as the value *9 for CH 2 ; while in the M/3 expression 

 for the paraffins the term 9 which represents the volume of 

 H + His only about half of the 15'7 for CH 2 . Associated 

 with this peculiarity of the paraffins, there must be taken 

 another, namely, that methane, CH 4 , while chemically the 

 first of the paraffins, separates itself absolutely from the series 

 as regards molecular force, but not as regards M/3. In respect 

 of molecular force it behaves as an element, as is shown in the 

 " Laws of Molecular Force," and in the " Viscosity of Gases 

 and Molecular Force" (Phil. Mag. Dec. 1893) ; and its value 

 of (M 2 /)% namely 1*5, is only a half of the 3*1 which it would 

 be if it came within the scheme for the paraffins in Table IV. 

 Thus it appears to be necessary for at least two CH 2 groups 

 to be associated before each takes its characteristic value in 

 (M'Z)*; and this must be connected with the series-building 

 power of carbon. It should be noticed that the value of CH 2 

 in (M 2 Z)* depends only slightly on the mode of association of 

 the carbon and hydrogen atoms according to the usual 

 structural formulas, because isomeric substances have almost 

 always nearly equal values ; thus, according to the equation 

 M£ = 1190 X 10-%^, as t\ varies very slightly from one 

 isomer to another, we see that amongst isomers (M 2 Z)* varies 

 nearly as the square root of the boiling-point. Take pentane, 

 C 5 H 1?J as an instance : for normal pentane, CH 3 (CH 2 ) 3 CH 3 , 

 the boiling-point is 38° C, while for tetramethylmethane, 

 C(CH 3 ) 4 , it is 9°-5 C; so that (M 2 /f for the latter is 

 (282-5/311)*, or "953 times that for the former, which, 

 according to the scheme, is 6' 7; and, accordingly, there is 

 only a difference of *3, corresponding to the great difference 

 in structure attributed to these two pentanes. Thus it is not 

 the position of the two terminal H atoms in the paraffins that 

 determines their apparent large value of 2*2 ; and provi- 

 sionally we have to recognize two values for H + H, one 

 being 2*2, and the other a fraction of *9, and must leave the 

 hydrogen atom to be returned to after we have considered 

 some others. 



In the alkyl haloid compounds we must regard the halogen 

 atom as displacing one of these terminal H atoms, for the 

 value of each CH 2 group in (M 2 Q* still remains the same. 

 Hence if we assume that the one terminal H left has a value 

 1-1 in (M 2 /) 1 and 4*5 in M/3, we can calculate at once from 

 ihe schemes in Table IV. the values of the halogen atoms in 



