﻿Dr. A. Crum Brown on Chemical Constitution. 399 



number of instances, the change of chemical structure produces 

 nearly the same change of boiling-point. These "laws"ofKopp 

 are only approximate, and are not even approximate in the cases 

 where the boiling-points of the substances compared are very dif- 

 ferently changed by change of pressure. 



Turning to the other physical character which has been mentioned, 

 namely, colour, we see at once a marked regularity. As a rule, 

 substances belonging to the same series differ from one another in 

 degree rather than in kind of colour ; while in passing from one 

 series to another, we observe that the colour undergoes a total change 

 of character. This is well illustrated by comparing the colours of 

 substances belonging to such series as the ferrous salts, the ferric 

 salts, the ferrates — the manganous salts, the manganic salts, the 

 manganates and the permanganates — the cupreous and cupric salts, 

 the chromous and chromic salts, the chromates and perchromic acid. 

 Possibly such changes of colour as we see in the transformation of 

 rosaniline and its derivatives into leukaniline and analogous bodies, 

 and of blue into white indigo, may be cases of the same kind. It is 

 also interesting to note that, while the nitro-substitution products 

 of the aromatic series are generally yellow, all the known substances 

 of the same kind in the fatty series are colourless. 



These considerations of colour would naturally incline us to re- 

 gard the operations which lead from one series to another as dif- 

 ferent in kind from those which lead from one member to another of 

 the same series ; and when we examine the physiological action of 

 bodies of the same and of different series, this impression is greatly 

 strengthened. 



The speaker described in some detail a few of the observations 

 made within the last two years by Dr. T. R. Fraser and himself, 

 pointing out the similarity of the action of substances belonging to 

 the same series, and the remarkable change of physiological action 

 produced by those chemical changes which lead from one series to 

 another. The illustrations w r ere drawn from the- natural alkaloids 

 (a group of substances containing trebly related nitrogen), and those 

 derivatives of the alkaloids which contain fivefold-related nitrogen. 

 It was shown that the salts of the alkaloids, although containing 

 fivefold-related nitrogen, were not adapted for this comparison, on 

 account of the readiness with which they lose acid in the presence 

 of alkaline substances, their nitrogen thus returning to the trebly 

 related condition. The bodies formed by the addition of a com- 

 pound of methyl have not this disadvantage ; and as the nitrogen 

 in them is permanently fivefold-related, their physiological action 

 may be satisfactorily compared with that of the alkaloids them- 

 selves. 



The experiments leading to a knowledge of the action of strychnia 

 and of the salts of methyl-strychnium were described ; and it was 

 shown that while the former acts by exciting the origins of the motor 

 nerves in the spinal cord, the latter act by diminishing the action and 

 ultimately paralyzing the terminations of the same nerves in the 

 muscles. Similar relations exist between brucia and methyl-brucium, 

 thebaia and the salts of methyl- thebaium, morphia and the salts of 



