PROCEEDINGS OF THE OHIO ACADEMY OF SCIENCE. 377 



knowledge and at the present time gives us by far the most sat- 

 isfactory conception of solutions that we have." 



The significance of the term "solution" has been greatly 

 extended in recent years so that now no restrictions are placed 

 on the physical state of the solution or its constituents. It is 

 logical therefore, to speak of solutions of gases in gases or of 

 solids in solids. Members of the latter type are known as solid 

 solutions and the study of their characteristics from the stand- 

 point of solutions has thrown much light upon a number of 

 intricate problems. For example, certain varieties of iron and 

 steel may be regarded as solid solutions of different carbides of 

 iron as well as of graphite in the pure iron — a view which has 

 been of great value in accounting for the characteristic properties 

 possessed by various grades of iron and steel. 



Colloids. Graham, for a long time Professor of Chemistry 

 in the University College of London, as an outcome of his studies 

 on diffusion divided all substances in solution or apparent solution 

 into two classes ; the one class, termed the crystalloids, easily 

 crystallize and rapidly diffuse through porous membranes; the 

 other known as colloids do not crystallize and diffuse only at a 

 very slow rate. Graham's work was published in 1862. For a 

 long time but little attention was given to it. The discovery, 

 however, that many of the constituents of living organisms are 

 colloidal in character drew a large number of workers into this 

 field of investigation. The results obtained are of the greatest 

 interest both from a theoretical and practical standpoint. 



A study of the so-called colloidal solutions showed that the 

 properties of many of these could only be accounted for upon 

 the supposition that in them we are dealing not with true solutions 

 but rather with suspensions ; in other words, the colloid present 

 is not in solution but in a very fine state of division — so fine 

 that not only is it invisible under ordinary conditions but that it 

 may remain suspended in the liquid indefinitely. We now know 

 that no sharp line can be drawn between colloids and crystalloids 

 since even the same substance can often be obtained in all tran- 

 sition stages from the distinctly coUoidad state to the distinctly 

 crystalloid. 



