Law of Density -Numbers* 



193 



Methyl glycerine, CH(OH) 3 , is not known, nor is methyl 

 erythrite (methylphycite), C(OH) 4 . But there are two com- 

 pounds which have the composition of the ethylines of these 

 two unknown bodies, and a third, which is the allyline of the 

 former : — 



(1) C 7 H ]6 3 = CH(OC 2 H 5 ) 3 , 



(2) C 9 H 20 O 4 = C(OC 2 H 5 ) 4? 



(3) C 10 H 16 O3 = CH(OC 3 H 5 )3. 



And these have all the same value of Tn/a, x = 7. This pro- 

 perty (the constancy of x) is to be met with in very many 

 other cases. I ought, however, to observe that in reality this 

 constancy of x is but a special case. When substitution - 

 products are formed each has a particular value of x, which 

 may be the same as that of the body from which it was formed, 

 or may be greater or less than that by one or more units, 

 according to circumstances. 



In instances where the value of x for the original substance 

 cannot be determined, it is frequently found that the products 

 of substitution, or some of them, have the same x. This is 

 the case with urea, CO(NH 2 ) 2 . According to Table I., the 

 value of B is 12, Table XVI. contains four substituted ureas; 

 # = 16 in three cases, but in the fourth = 11; or a difference 

 of 5 — a number which we shall find later on is a very frequent 

 difference in the value of x for analogous bodies. In the 

 following table I have, instead of the constant TB/a, inserted 

 the value of #, =1(27*8) 2 x (TB/a) 2 , which is the same thing, 

 but in this instance more convenient. The boiling-points are 

 the mean of those given in Fehling's dictionary. 



Table XVI. 



Name. 



Formula. 



A. 



B. 



S°, 

 observed. 



found. 



X, 



calculated. 



Urea 



CO(NH 2 ) 2 ... 

 CO(^HCH 3 ) 2 



60 



19 









Dimethyl urea . . . 



88 



18 



269 



15-90 



16 



Methylethyl urea. 



00 NHOH 3 

 uu NH0 2 H 5 - • 



102 21 



267 



15-99 



16 



Diethyl urea 



CO(NHC 2 H 5 ) 2 



116 24 



263 



1591 



16 



Tetramethyl urea. 



CO(N(CH 3 y 2 



116 



24 



176 



11-16 



11 



In the last column of the table is given the theoretical value 

 of x, which is simply the whole number nearest to the found 

 Phil Mag. S. 5. Vol. 20. No. 123. August 1885. 



