552 SCIENCE PROGRESS 



gum arabic, dextrin, cream of tartar, Rochelle salt, saltpetre and 

 potassium sulphate. 



D. Morse's Experiments 



The experiments on osmotic pressure which have been 

 conducted at the Johns Hopkins University by Prof. H. N. 

 Morse and his co-workers have formed the subject of twenty-five 

 papers published in the American Chemical Journal from 1901 to 

 191 1. But most of the essential features of the earlier papers 

 are described, with methods perfected and data corrected, in a 

 series of five papers which appeared in that journal in 191 1 

 under the heading " The Relation of Osmotic Pressure to 

 Temperature." These five papers will long stand as one of the 

 monuments of Physical Science and may already be ranked 

 with the great classics of earlier generations. A sixth paper 

 dealing with the " Osmotic Pressure of Cane-Sugar Solutions 

 at High Temperatures " has appeared during the past year and 

 a further paper on this subject is promised. It will be con- 

 venient to describe in series the chief features of the apparatus 

 which enabled the American workers to reduce the measurement 

 of osmotic pressure from a rough approximation to an exact 

 routine. 



1. The Manufacture of the Cells. — One of the most serious 

 difficulties in the measurement of osmotic pressure is to secure 

 suitable porous pots. This difficulty was encountered by Pfeffer 

 but became of dominant importance in the more exact work of 

 the American investigators. At the end of four years they had 

 secured (from a batch of 100) only two cells with which they 

 could measure osmotic pressure with some degree of confi- 

 dence, whilst 25 or 30 answered the requirements moderately 

 well. A whole year spent in procuring and testing nearly 500 

 more cells from different makers revealed not one that was 

 suitable for the work and showed that the problem must be 

 transferred from the pottery to the laboratory. 



The chief faults of the commercial cells were : 



(1) Insufficient strength : only a few survived 30 atmospheres, 



whilst most of them cracked at pressures below 20 

 atmospheres. 



(2) " Air-blisters," communicating with each other and with 



the interior of the wall, which gave rise to a series of 

 subsidiary membranes in the interior of the wall, 



