64 MOLECULAR CONSTITUTION 



this class a compound radicle, such as cyanogen (CN) or ammonium 

 (NH4), may replace one of the simple metallic elements, and that 

 two such salts may combine together to constitute a double salt ; 

 or that the metallic element may be replaced by a more complex 

 radicle, such as urea (C^N'^H40=), or kreatinine (CgH^NjO^), or even 

 by one of the still more complex bodies known as alkaloids,' we 

 may be somewhat amazed at the marvellous atomic complexity 

 which is to be attained even by the crystallisable bodies known as 

 salts. 



What has been said concerning crystallisable bodies obtains also 

 with regard to the compounds known as ' colloids.' In this class 

 are included all the plastic elements of animals and plants — that 

 is, the various protein compounds. They are supposed to be 

 generally characterised by the large size and complexity of the 

 molecules of which they are compounded. Thus Prof. Graham 

 says " : — " It is difficult to avoid associating the inertness of colloids 

 with their high equivalents, particularly where the high number 

 appears to be attained by the repetition of a smaller number. 

 The enquiry suggests itself whether the colloid molecule may not 

 be constituted by the grouping together of a number of smaller 

 crystalloid molecules, and whether the basis of coUoidality may not 

 really be this composite character of the molecule." 



No hard and fast line, however, separates the colloids from the 

 crystalloids. Although multitudes of bodies exist which may be 

 easily placed in one or the other class, multitudes of others are to be 

 met with having properties of an altogether intermediate character. 

 Nay, even the most typical colloids may undergo a rearrange- 

 ment of their elements, whereby they are converted into crys- 

 talloids. Nothing could show more plainly than this, that the 

 difference between a crystalloid and a colloid is merely one of 

 degree, and that the properties of colloids are different merely 

 by reason of the more complex molecular arrangement which 

 prevails — an arrangement, however, which, by the mere influence 

 of physical conditions, the molecules of certain crystalloids are 

 known ' spontaneously ' to assume. A further consequence directly 

 flowing from this superior complexity of the colloid molecules, 

 is, the greater instability which characterises them as a class. 



' The composition of narcotine, for instance, is said to be C^sH^s N0'<, and 

 that of morphine, C^H^NOs + 2HO. Both bodies have distinct crystalline 

 forms. 



' Phil. Trans., 1861. 



