CHEMICAL SCIENCE. 2^9 



diffusible in water he terms crystalloids ; the solution of these is always 

 free from gumminess or viscocity, is sapid, possessing in a higher or 

 lower decree the power of affecting the nerves of taste. The other 

 class, whose dilFusive power is low, he distinguishes as colloid* , be- 

 cause gelatin or glue (co//<?) maybe taken as their type. The solu- 

 tions of these substances have no disposition to crystallize, and in 

 the solid form they do not possess flat surfaces, such as characterize 

 crystals, but exhibit an irregular roundness of outline. Their solu- 

 ti;uis are always gummy when concentrated, and what is strikingly 

 remarkable, they are all insipid or wholly tasteless. In the moist 

 condition they are liable to undergo great changes, and solutions of 

 them in a state of purity cannot be preserved unaltered for any length 

 of time. 



A solution of a colloid body, such as gelatin, is found to offer 

 scarcely any impediment to the diffusion of a crystalloid throughout 

 its entire mass. This diffusion will also take place through any soft 

 solid with almost equal rapidity ; a very familiar example of this fact 

 is shown in the process of salting meat, in which case the rapidly dif- 

 fusible crystallizable sea-salt penetrates to the interior of the flesh, 

 which is a combination of different colloid bodies, such as fibrin, albu- 

 men, gelatin, etc. 



Upon the fact that crystalloid bodies possess the power of diffusing 

 themselves through soft solids depends the operation known as dialy- 

 sis, and the construction of the instrument called the dialyser. This 

 consists simply of a tambourine-shaped frame of gutta-percha, over 

 which is tightly stretched a piece of parchment paper, which completes 

 the resemblance to that musical instrument. This parchment paper 

 is quite impervious to water, so that no passage of fluid similar to 

 nitration can take place through it. If the dialyser be floated on 

 the surface of pure water, and a mixed solution of a crystalloid and a 

 colloid body be poured into it, the process termed dialysis immedi- 

 ately commences ; all the crystalloid matter passes through the parch- 

 ment paper into the water, and the colloid matter remains behind in 

 the dialyser. As an instance of its action, let us suppose a mixed 

 solution of sugar and gum to be poured into the dialyser, when the 

 sugar passes through into the water below, and the gum remains be- 

 hind in a pure form. If a mixture of the beautiful aniline dye known 

 as magenta, and some burnt sugar or caramel be employed, the pass- 

 age of the magenta into the pure water is readily observed, the dark- 

 brown uncrystallizable colloid caramel remaining in the dialyser. 



Other facts of great interest have been discovered as the results of 

 these investigations. Thus it is found that by means of dialysis, 

 we may obtain pure in solution many substances hitherto regarded as 

 being perfectly insoluble. Amongst these may be mentioned silica, 

 alumina, Prussian blue, peroxyd of iron, stannic acid, and numerous 

 other bodies of a similar character. 



For example, if a solution of soluble glass, which is formed by fus- 

 ing silica with an excess of soda, be taken and acidified with hydro- 

 chloric acid, the acid unites with the soda, forming common salt, or 

 chloride of sodium, the silica remaining for some time dissolved in a 

 gelatinous or colloid form, mixed with the solution of the chloride of 

 sodium. If, however, this mixture of gelatinous silica and common 



