i LIVING MATTEE 21 



These figures, of course, throw no light on the grouping of the 

 respective elements; i.e. the chemical structure of the protein 

 molecule. They show, however, that the different proteins form 

 a well-defined class of chemical compounds, having a strict 

 relation among themselves, as is further apparent from the 

 physico-chemical properties common to the several members, as 

 follows : 



(a) Non-diffusibility through the pores of animal or vegetable 

 membranes and of artificial parchment ; they belong, therefore, to 

 the class of bodies which Graham termed colloids. They are 

 obtained in a crystalline form with difficulty, and only by special 

 methods. If the colloid is fluid it is termed sol; if solid, gel. 

 Liquid and solid gelatin are examples of these two states. 

 When water is the medium in which the colloid is dispersed the 

 terms hydrosol and hydrogel are used respectively. Besides the 

 proteins, many inorganic substances can exist in a colloidal form, 

 e.g. colloidal metals, silicic acid, etc. There has been much recent 

 discussion as to the state in which the colloids exist in a solvent 

 (which in the case of the proteins of the living body is exclusively 

 represented by water). According to the latest conclusions, we 

 are here concerned not with true solutions having the well-known 

 properties of solutions, due to the mixing of the soluble crystalloids, 

 salts, urea, glucose, etc., with water but rather with very fine 

 emulsions or suspensions, i.e. the particles of the colloid substance 

 can be seen in a separate state, suspended in the liquid, and do 

 not enter into those intimate relations with the solvent on which 

 depend the physico-chemical characters of true solutions (osmotic 

 pressures, homogeneity under high magnification, etc.). In fact, 

 these colloidal solutions scarcely lower the freezing-point of the 

 solvent, and under the ultra-microscope a^re seen to consist of 

 various-sized granules moving in the body of the fluid. 



(6) All proteins have, further, very definite chemical properties, 

 by which they are sharply differentiated from all other known 

 chemical aggregates, crystalloids or colloids. Their aqueous 

 solutions are optically active, since they deflect the plane of 

 polarised light to the left. Heat, the addition of small quantities 

 of mineral acid, salts of the heavy metals, as also absolute alcohol, 

 solutions of tannin, phosphotungstic acid, picric acid, etc., pre- 

 cipitate and often coagulate them (albumins and globulins). In 

 this case the protein molecule undergoes profound changes, for 

 after removal of the precipitating agfent the initial state of col- 

 loid cannot be restored ; the protein is said to be de-natured. 

 Proteins are further precipitated by saturation of the solvent with 

 salts of the alkalies or alkaline earths (sodium chloride, magnesium 

 sulphate, ammonium sulphate). It is important in the chemistry 

 of the proteins to note that in the precipitation determined by 

 these salts the proteins are not de-natured, or at any rate become 



