260 



SCIENCE 



[N. S. Vol. XXXIV. No. 870 



separated by the "mechanical operation" 

 solution. And yet, every high school 

 chemistry gives instances of solid chemical 

 compounds decomposed by water, and 

 some even speak of the hydrolysis of salts 

 like bismuth nitrate, which can be formed 

 by bringing together bismuth hydrate and 

 nitric acid of the proper concentration and 

 from which by treatment with water all 

 the nitric acid can be removed, leaving the 

 bismuth hydrate behind. Such instances 

 of self-contradiction (where the major 

 premise of some argument is quietly nega- 

 tived elsewhere in the book) are scattered 

 broadcast. "In spite of the most careful- 

 ness," to quote from the advertisement of 

 a new German balance, "the rider will 

 fall ' ' ; and the most carefulness has cer- 

 tainty been employed in boolcs which use 

 syrup instead of brine in their illustra- 

 tions out of respect for the ions, and which 

 the Roozeboom diagram has driven to sub- 

 stitute platinum for iron in the old mis- 

 leading statement that "powdered iron, 

 magnetized iron, glowing iron and melted 

 iron are just as truly iron" as a cold 

 poker. For one illustration is as bad as 

 another if it is used to confound solutions 

 with mechanical mixtures, or to obscure 

 the fact that chemical properties change 

 with the temperature, and in some cases 

 are measurably affected even by fine grind- 

 ing. 



The vagueness of the texts, already re- 

 ferred to, serves to keep their self-contra- 

 diction in a measure hidden. If every 

 statement is indefinite, all can be recon- 

 ciled ; and what could be less definite than 

 the customary definition of an element as 

 a substance from which "nothing simpler 

 than itself" can be obtained, if the mean- 

 ing of "simpler" is left to be guessed? 

 Unless perhaps it be the definition of a 

 molecule as "the smallest quantity of a 

 compound that can exist alone," or the 



corresponding indefinitions of atom, affin- 

 ity, stability, valence, solvent and solute. 



This vagueness is, naturally, most 

 marked just where clearness of thought 

 and precision of statement would show the 

 untenability of the exceptional position 

 assigned to the chemical individual; so 

 that it is hardly surprising to find that 

 not one of the texts gives a working defi- 

 nition («'. e., one that can be applied in 

 the laboratory) of the very group of sub- 

 stances which figures so prominently in all 

 of them. 



One of the Best Sellers defines chemical 

 changes as "those which involve a change 

 in the composition of the matter," while 

 "sugar may be dissolved in water, but 

 neither the sugar nor the water is changed 

 in composition." This book, like the rest 

 of them, brings in all the usual extra-ob- 

 servational hypotheses to "explain" the 

 ordinary "laws of chemical combination," 

 but gives no explanation whatever of this 

 most extraordinary use of the word "com- 

 position"; although on this in definition 

 of chemical change is based the definition 

 of chemical compound, that of mechanical 

 mixture, and by implication that of chem- 

 ical affinity as well. 



Let us enquire what significance is at- 

 tached in the practise of the present time 

 to the terms mechanical mixture and 

 chemical compound; we shall then be in a 

 position to appreciate the difficulty in 

 which the text-books find themselves and 

 from which they seek to escape by the em- 

 ployment of systematic mystification as an 

 aid to teaching. 



Mechanical mixtures which for years 

 have posed in the pages of Dammer as 

 chemical compounds, are by the applica- 

 tion of the phase rule daily being removed. 

 What criterion has been adopted in each 

 of these cases? Stripped of technical 

 terminology it is: Whenever the reacting 



