498 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1912. 



But the line between inorganic and organic chemistry, which up to 

 the middle of the last century appeared sharp, subsequently became 

 misty and has now disappeared. Similarly the chemistry of H"ving 

 organisms, which is now a recognized branch of organic chemistry^, 

 but used to be considered as so much outside the domain of the 

 chemist that it could only be dealt with by those whose special 

 busmess it was to study "vital" processes, is passing more and more 

 out of the hands of the biologist and into those of the pure chemist. 



THE COLLOID CONSTITUTION OF LIVING MATTER. 



Somewhat more than half a century ago Thomas Graham published 

 his epoch-making observations relating to the properties of matter in 

 the colloidal state, observations which are proving all-important in 

 assisting our comprehension of the properties of living substance. 

 For it is becoming every day more apparent that the chemistry and 

 physics of the Hving organism are essentially the chemistry and 

 physics of nitrogenous colloids. Living substance .or protoplasm 

 always, in fact, takes the form of a colloidal solution. In this solu- 

 tion the colloids are associated with crystalloids (electrolytes), which 

 are either free in the solution or attached to the molecules of the 

 colloids. Surrounding and inclosing the hving substance thus con- 

 stituted of both colloid and crystalloid material is a film, probably also 

 formed of coUoid, but which may have a Upoid substratum associated 

 with it (Overton). This film serves the purpose of an osmotic mem- 

 brane, permitting of exchanges by diffusion between the colloidal solu- 

 tion constituting the protoplasm and the circumambient medium ia 

 which it lives. Other similar films or membranes occur in the interior 

 of protoplasm. These films have in many cases specific characters, 

 both physical and chemical, thus favoring the diffusion of special kinds 

 of material into and out of the protoplasm and from one part of the 

 protoplasm to another. It is the changes produced under these 

 physical conditions, associated with those caused by active chemical 

 agents formed within protoplasm and known as enzymes, that effect 

 assimilation and disassimilation. Quite similar changes can be pro- 

 duced outside the body {in vitro) by the employment of methods of 

 a purely physical and chemical nature. It is true that we are not yet 

 famihar with aU the intermediate stages of transformation of the 

 materials which are taken in by a living body into the materials which 

 are given out from it. But smce the initial i^rocesses and the final 

 results are the same as they would be on the assumption that the 

 changes are brought about in conformity with the known laws of 

 chemistry and physics, we may fairly conclude that all changes in 

 h^^ng substance are brought about by ordinary chemical and physical 

 forces. 



