292 Mr. T. Graham on Liquid Diffusion applied to Analysis. 



was conveniently applied to dialytic experi- | g ' ' 



ments. Two sizes of the bulb were employed, — ^ ™- 

 3* 14 and 4*44 inches in diameter respec- 

 tively, and of which the dialytic septa pos- 

 sessed an area very nearly of y^-th and 

 o-Jrxrth of a square metre (15*6 and 7*8 

 square inches). With 100 cub. centims. of 

 fluid in the osmometer (the volume usually 

 employed), the septum of the smaller instru- 

 ment was covered to a depth of about 20 

 millimetres (0*8 inch), and the septum of 

 the larger to a depth of 10 millimetres 

 (0*4 inch). The thinner the stratum, the jgj 

 more exhaustive the diffusion in a given 

 time. It is generally unadvisable to cover the septum deeper 

 than 10 or 12 millimetres (half an inch), where a considerable 

 diffusion is desired within twenty-four hours. The following 

 practical observations maybe found useful in applying the dialyser 

 to actual cases of analysis. They refer to the parchment-paper 

 septum, which is much the most convenient for use. 



With a 2 per cent, solution of chloride of sodium, containing 

 2 grammes of the salt, and covering a septum of nearly 0'01 

 square metre (15*6 square inches) in area, to a depth of 10 milli- 

 metres, the salt which diffused in five hours amounted to 0*75 

 gramme, and in twenty-four hours to 1 # 657 gramme, leaving 

 behind 0*343 gramme, or 17*1 per cent, of the original salt. 

 The following experiments, made with the same osmometer and 

 solution, show the effect of reducing the volume of liquid placed 

 in the dialyser. The proportion of salt which diffused out in 

 twenty-four hours was — 



From 100 cub. centims. of solution 86 per cent. 

 From 50 cub. centims. of solution 92 per cent. 

 From 25 cub. centims. of solution 96 per cent. 



In all cases the volume of water outside into which the salt 

 escaped was ample, being from five to ten times as much as the 

 volume of fluid placed in the dialyser, and it was changed during 

 the continuance of the experiment. A much less volume of ex- 

 ternal water suffices, provided it is changed at intervals of a few 

 hours. The temperature was 10° to 12°. It will be observed 

 that these volumes correspond to a depth of liquid in the dialyser 

 of 0*4, 0*2, and 0*1 inch respectively. 



The time of travelling through the thickness of the parchment- 

 paper itself may be observed, and is worthy of remark. 



Of the quality of parchment-paper always used in these experi- 

 ments, as quare metre, when dry, weighed 67 grammes, and when 

 charged with water 108*6 grammes. Taking the specific gravity 

 of cellulose at 1 *46, that of the lighter woods, the parchment-paper 

 described will, in the humid state, have a thickness of 0*0877 



