BY DR. LAUDER BRUNTON. .">;."> 



serves as a pipette to remove a little of the fluid occasionally from the fell 

 for the purpose of testing it. The cell is placed in a glass cylinder, little 

 more than wide enough to admit it, and the fluid to be tillered is poured 

 into the cylinder until it (-overs the lo\ver part of the India-rubber rap. 

 The air being then exhausted from the cell, the fluid filters into it from 

 the cylinder. Instead of cells, cones of porous earthenware may be used 

 as tillers. A short piece of wide India-rubber tubing is stretched over 

 the top of a funnel, and into its upper end, which lies flat across the 

 opening of the funnel, a porous cone is inserted (see fig. 335). In order 

 to keep liquids hot during the process of filtration, Plantamour's funnel 

 is used. This is a hollow funnel of copper containing water, which is 

 kept hot by a flame applied to a projecting part. A better plan is to 

 use a water-bath with a funnel-shaped opening in it (fig. 336). This 

 has the advantage that it may be kept at any required temperature with 

 the aid of a Bunsen's regulator. 



212. Dialysis. Almost all crystalline bodies, with the notable ex- 

 ception of hemoglobin, pass readily, when in a state of solution, through 

 animal membranes or through vegetable parchment. The great ma- 

 jority of non-ciystalline bodies, such as albumin, do not pass through at 

 all, or only with very great difficulty. In this way the diffusible may 

 be separated from non-diffusible substances. Such a separation is 

 termed dialysis. Graham, the discoverer of the process, gave to the 

 ditl'usible bodies the name crystalloids, to the non-diffusible the name 

 colloids, as he thought all crystalline bodies diffused and all non-crystal- 

 li/ablo did not ; but these names are open to- objection since the dis- 

 covery that hemoglobin will not diffuse, although it forms crystals, 

 while peptones diffuse, although they do not crystallize. Dialysis is 

 effected by placing the liquid which is to be dialysed in a cylinder, of 

 which the bottom consists of vegetable parchment. This cylinder, 

 called a dialyser. is then placed in a shallow vessel containing distilled 

 water. The diffusible substances pass through the parchment into the 

 water, while the non-diffusible remain behind. Two forms of dialyser 

 are in ordinary use. For dialysing small quantities, bell-shaped glass 

 jars are used. For quantities of seven or eight ounces or upwards, a 

 dialyser is employed which consists of two gutta-percha hoops, one of 

 which is two inches deep, the other only one. The deeper hoop is 

 slightly conical, so that the other hoop slips over its smaller end. 



Before using this contrivance, both hoops must be washed thoroughly 

 with distilled water. A piece of vegetable parchment, about three 

 inches wider than the smaller end of the deep hoop, must then be 

 steeped for a minute in distilled water and stretched over it. After 

 applying the edges of the parchment carefully to the outside of the 

 smaller hoop, the larger one is slipped over it, so as to fix it tightly. 

 The dialyser musi next b" tested, to ascertain that the parchment is 

 free from holes. It must be filled to the depth of a quarter of an inch 

 with distilled water, and placid for a short while on a piece of blotting- 

 paper. If there are any holes in the parchment, the water will come 

 through and leave a wet spot on the blotting-paper, in which case either 

 a fresh piece should be put on or the hok's closed up. This may be 

 done by sticking a piece of vegetable parchment over the holes on the 

 under surface of the dialyser \vith white of egg, and then passing a 

 smooth hot iron over the patches. This done, the dialyser must be 

 again tested. After having been ascertained to be perfect, it may be 

 filled ; the liquid to be dialysed must not cover the bottom to a greater 

 depth than half an inch. It must then be floated in about five times as 

 much water as it contains of liquid (fig. 337), and gently agitated from 

 time to time. 



The bell-shaped dialysers are used in the same way, but the paper is 



