CELLS AND TISSUES. 25 



between our imagination and fact, due in part to a lack 

 of chemical data of biological importance, in part to a 

 lack of that theoretical foundation necessary for special 

 questions in biology. For this reason the following 

 fragments of a general presentation of the physical 

 chemistry of the cells and tissues cannot claim to be 

 complete or to give a satisfactory account of facts to 

 which nothing more will ever be added. It must suffice 

 if the great importance of physical chemistry in general 

 physiology is rendered apparent. 



All living matter is made up of colloidal and crystal- 

 loidal material, and there exists no life process that is 

 not accompanied by changes in the colloidal and crystal- 

 loidal substances. And the physico-chemical laws which 

 govern the crystalloids and the colloids reappear in the 

 numerous properties of living matter. 



The colloids have for the most part a high molecular 

 weight, diffuse only with the greatest difficulty, and do 

 not pass through animal membranes. Solutions of col- 

 loids have a scarcely measurable osmotic pressure, and 

 have in consequence little effect in raising the boiling- 

 point or depressing the freezing-point. They do not con- 

 duct the electric current, yet they move, for the most part, 

 in an electric current. 



The crystalloids diffuse easily and pass readily through 

 animal membranes. Their molecular weight is low, 

 while their affinity for water, as measured by an in- 

 crease in the boiling-point or a depression of the freezing- 

 point of their solutions, is very great. The crystalloids 



