654 



SCIENCE 



[N. S. Vol. LIV. No. 1409. 



sodium which were in free solution but which 

 are changed in the absorbing' medium to a 

 colloidal state m. which they are extremely 

 inactive and in which state they fail to re- 

 spond to reagents that would normally reveal 

 their presence. It is a matter of very great 

 diificulty, therefore, to determine whether the 

 electrolytes found in the ultimate analysis of 

 the ultra-clay are constitutionally combined 

 with the silicate of alumina and iron, or 

 whetlier they merely exist in a passive col- 

 loidal state. 



THE TRUE SOLUTION 



The study of ulti'a clay includes funda- 

 mental problems of physical chemistry, partic- 

 ularly the differences in state between true 

 solutions and colloidal solutions. The 

 modern concept of solutions ascribes a rather 

 complex form to the molecule of water. The 

 molecule of a salt dissolved in water forms 

 numerous and indefinite hydrates with the 

 surrounding water molecules. The complex- 

 ity of these hydrates is influenced by con- 

 centration and by temperature, or in other 

 words by the balance in the internal energy 

 of the system expressed by the activities of 

 the water molecules on the one hand and the 

 activities of the salt molecules on the other, 

 as well as the activity or energy exerted be- 

 tween the water molecules and the salt 

 molecules. 



Little is known about the complexity of the 

 hydrates in the solution. We have actual data 

 only when certain salts crystallize from the 

 solution. With salts that crystallize with 

 water of hydration the lowest hydrates are 

 formed at the higher temperatures and the 

 higher hydrates are formed at the lower 

 temperatures. Magnesium chloride is known 

 to crystallize with five different amounts of 

 water, namely, 2, 4, 6, 8, and 12 H^O. The 

 higher hydrate (12 11,0) separates at tem- 

 peratures between 16.8° to —33.6° C. The 

 lowest hydrate comes out at 181.5° C. 



Sodium carbonate is known with three 

 states of hydration, namely, 1, 7, and 10 H,0, 

 depending upon the temperature. If inter- 

 mediate hydrates occur they do not appear 



to be stable forms. It would appear there- 

 fore that 12 H„0 is the highest stable hydrate 

 formed in crystals except for the double mole- 

 cules of the alums which carry twice this 

 amount or 24 H„0. Sodium chloride crys- 

 tallizes out at ordinary temperatures without 

 water of hydration, the crystal being com- 

 pletely dry on the inside. It is said to crys- 

 tallize with 2 HjO at temperatures somewhat 

 below zero. Sodium sulphate crystallizing at 

 room temperature immediately changes under 

 the same temperature conditions to the an- 

 hydrous form when the solution becomes sat- 

 urated with sodium chloridfe. This gives us 

 a vision of only a few proportions of water 

 which fit into the molecular structure of the 

 crystal in permanent form. It throws no 

 other light wpon proportions of water of hy- 

 dration which may occur in solution which 

 would not fit into the structure of a crystal 

 in stable form. 



The strength of these hydrates differs 

 markedly. Sodium carbonate with 10 H^O on 

 exposure to air is reduced to sodium carbon- 

 ate with 1 11,0. Sodium sulphate with 10 

 HjO gives off free water under pressure as 

 when pestled. Any of the three known hy- 

 drates of calcium chloride, namely, 2, 4, 6 

 HjO, absorbs water when exposed to ordi- 

 nary air, which changes from a gaseous to a 

 liquid state in which the calcium chloride 

 finally dissolves. 



It appears therefore that the water of hy- 

 dration is influenced by the internal energy 

 of the system and may be modified by the 

 external pressure of the water vapor in the 

 air and by mechanical force. 



There are of course numerous cases where 

 salts crystallize without water of hydration 

 or where they come down in the form of 

 amorphous material without crystalline form. 

 There are likewise numerous cases where it is 

 difficult to secure crystals and often impos- 

 sible to separate material in a solid state 

 which ordinarily comes out in a solid form. 

 The difficulties in obtaining sugar in a solid 

 form with certain impurities, particularly 

 glucose and potassium, or iron, is a case in 

 point. The impossibility of obtaining ortho- 



