CHAMBERS'S INFORMATION FOR THE PEOPLE. 



bladder is as salt as that within ; and if the water 

 outside be frequently renewed, the whole of the 

 salt can be removed from the bladder. If, instead 

 of a solution of salt, we take a solution of glue 

 (weak enough to prevent its setting as a jelly), we 

 find that practically none of the glue escapes into 

 the outer vessel If a mixture of salt and glue be 

 treated in this way, the two substances will be 

 separated the salt passing through the bladder, 

 and the glue remaining behind. Many substances, 

 especially those which crystallise, behave like salt, 

 and pass through the bladder ; while many other 

 substances, especially those which, like glue, do 

 not crystallise, but form jellies, and, when dried, 

 appear as horny masses, pass extremely slowly 

 through the bladder. The late Mr Graham, 

 Master of the Mint, who discovered this method, 

 called the first set of bodies ' crystalloid ' that is, 

 crystal-like; and the second, 'colloid' that is, 

 glue-like bodies. Dialysis is used in detecting 

 poisons in animal fluids, such as the contents of 

 the stomach. Most of the important poisons are 

 crystalloid ; while most of the animal fluids that 

 interfere with the detection of poisons are colloid, 

 so that the two can be separated by this means. 

 Dialysis has also been applied to remove the salt 

 from the brine in which meat has been salted. 

 Such brine contains much nutritious matter dis- 

 solved out of the meat, but is far too salt to be 

 used as food ; by dialysis the salt can be removed, 

 while the nutritious matter, being colloid, remains 

 in the bladder. 



5. Fusion. A mixture of a fusible and an infus- 

 ible substance may be resolved into its ingredients 

 by heating it till the fusible body melts, and then 

 pressing it, or allowing it to drain away from 

 the infusible residue. The same method may be 

 employed to separate from one another two sub- 

 stances of different degrees of fusibility, by heating 

 the mixture to a temperature higher than the 

 fusing-point of the one, and below the fusing-point 

 of the other. 



6. Distillation. Some substances are volatile 

 that is, can be converted by the action of heat 

 into vapour, which, when cooled, is condensed, or 

 restored to the liquid or solid state. By distilla- 

 tion we can separate volatile from non-volatile 

 bodies. The mixture is heated in a vessel (retort 

 or still), to which a tube is adapted, to convey the 

 vapour to the ' condenser,' where it is cooled. The 



\L 



Fig. I. 



volatile substance distils over, while the non- 

 volatile ingredient remains in the retort or stilL 

 By what is called fractional distillation, it is 

 possible to separate, more or less completely, sub- 

 stances of different degrees of volatility ; the first 



206 



portion of the distillate contains a larger propor- 

 tion of the more volatile ingredient, and the last 

 a larger proportion of the less volatile. Thus, 

 when the ' wash,' or liquid formed by the fermenta- 

 tion of malt, is distilled, the first portion consists 

 of a very volatile product called the ' foreshot ; ' 

 then alcohol mixed with water distils over ; and 

 lastly, a substance less volatile than alcohol, the 

 ' fousel oil.' 



7. Diffusion of Gases. If a heavy gas and a 

 light gas are placed together in a vessel, the heavy 

 gas below and the light above, they slowly mix, 

 even when left at rest ; but, when once mixed, they 

 shew no tendency to separate ; and however long 

 the mixture is kept, the mixture at the top will be 

 found to have the same composition as that at 

 the bottom. It is, therefore, often necessary to 

 apply a test, in order to determine whether a 

 given specimen is a single gas or a mixture. 

 Some of the methods already mentioned may 

 sometimes be employed for this purpose. Thus, 

 gases differ in their solubility, and in this way it 

 may be shewn that atmospheric air is a mixture, 

 one of its ingredients (oxygen) being considerably 

 more soluble in water than the other (nitrogen). 

 Again, some gases can be condensed into liquids 

 by cold and pressure, and thus partially separated 

 from those which cannot be so condensed. By 

 this means Faraday discovered in oil-gas the gas 

 now known as butylene. 



There is, however, a special method by which, 

 in almost all cases, mixtures can be distinguished 

 from single gases. 



The slow mixing of two gases, of which we have 

 just spoken, and which is known as ' diffusion ' of 

 gases, takes place not only when the two gases 

 are in immediate contact, but also when they are 

 separated by a porous septum (such as a plate of 

 unglazed earthenware), and in this case it is easy 

 to measure the ' rate of diffusion ' by observing 

 what quantity of gas passes through the septum 

 in a given time. Now, different gases have differ- 

 ent rates of diffusion the lighter the gas the 

 faster does it diffuse.* If, then, a mixture of two 

 gases of different density be allowed to diffuse 

 through a porous plate, more of the lighter gas 

 will pass out, and proof will be obtained that the 

 specimen was a mixture, and not a single gas. 



The various methods of which we have given 

 a general description are physical or mechanical 

 modes of separating completely or partially the 

 ingredients of a mixture, and thus, more or less 

 perfectly, obtaining pure substances. Let us 

 now consider some of the properties of these 

 pure substances. For this purpose, it is neces- 

 sary to classify them, or arrange them in groups. 

 This may be done in several different ways. 

 We may, for instance, distinguish them as, first, 

 simple ; and second, compound. A compound 

 substance is one which we can, by the appli- 

 cation of some process, separate into two or 

 more pure substances, different from each other 

 and from the original substance. We shall give 

 an instance, and shew how a compound differs 

 from a mixture. White statuary marble is a pure 

 body ; it cannot, by any mechanical process, such 

 as we have described above, be separated into in- 

 gredients any such mechanical mode of sampling 



* Experiment has shewn, in complete accordance with theory, 

 that the rate of diffusion is inversely proportional to the square 

 root of the density of the gas. 



