166 COMPOUNDS, OF THE PROTEINS 



sociated molecules can form "solvates," and that these hydrates or 

 "solvates" are readily decomposed at temperatures which approach 

 the boiling-point of the solvent, and by the presence of other agents in 

 the solution which compete for the solvent. The determination of the 

 quantity of water bound in this way by substances in aqueous solution, 

 is frequently a matter of difficulty and uncertainty, but the existence 

 of such "solvate" compounds may be demonstrated in a variety of 

 ways, although their quantitative composition remains, in general, 

 unknown. A very striking experiment which illustrates the formation 

 of " solvates" is that cited by Pickering. If a mixture of Propyl Alcohol 

 and water be placed in a semipermeable vessel and surrounded with 

 water, it is found that water enters the cell, but that no propyl alcohol 

 escapes. If, however, the same semipermeable vessel, containing the 

 same mixture of propyl alcohol and water, be immersed in propyl 

 alcohol, propyl alcohol enters the cell and water does not leave it. In 

 other words, the vessel is permeable to either propyl alcohol or water 

 when these are pure, but it is impermeable to mixtures of the two, the 

 inference being that large molecular complexes are formed on mixing 

 these reagents which cannot pass through the pores of the vessel. From 

 these and similar experiments Poynting concludes that osmotic pressure 

 is an expression of the diminution in the active mass of the solvent due 

 to the formation of compounds with the dissolved substance. 



It is a familiar fact to chemists that anhydrous Cobalt Chloride 

 is blue, but that on taking up water it becomes violet or red. Ostwald 

 believed that the undissociated cobalt chloride molecule is blue, while 

 the cobalt ion is red. Since, however, the color of a concentrated 

 solution of cobalt chloride can be changed from purplish-red to blue 

 by the addition of relatively small amounts of calcium salts, or still 

 smaller amounts of aluminium chloride, or by the addition of a few drops 

 of alcohol, it is more probable that this change in color is due to dehy- 

 dration of the cobalt chloride molecule in solution, by the abstraction 

 of water from it by the added substance. Similarly the progressive 

 change in color of Cupric Chloride solutions, from blue to greenish-brown, 

 on concentration or dehydration is attributed to the loss of water on the 

 part of cupric chloride water-complexes. G. N. Lewis finds that if 

 various bromides be added to concentrated solutions of Cupric Bromide 

 the copper salt is dehydrated (turned brown) by the salts of mono- 

 valent metals in the order: Li > Na > NEU > K. Divalent metals 

 dehydrate more strongly, the order being: Mg > Ca > Sr > Ba while 

 trivalent metals (Al) act still more energetically. The resemblance 

 between the order of effectiveness of the monovalent metals in dehy- 

 drating cupric bromide and their order of effectiveness in coagulating 

 "electronegative" protein is very evident. 



The peculiar interest to the biological chemist of the possibility thus 

 indicated, that substances dissolved in water form loose combinations 

 with the solvent, lies in the especial significance of water in relation to 



