APPLICATIONS OF CRYSTALLOGRAPHY, ETC. 575 



other and liquid compounds ; further, the crystalHne char- 

 acters of the soHds examined are scarcely ever specified. 

 The kind of misunderstanding to which this may lead is well 

 exemplified by the following case. The curious statement 

 has been on record for many years that the latent heat of 

 fusion of chloral hydrate prepared immediately before exami- 

 nation by melting and then soldifying is less than that of 

 the same material solidified some days before examina- 

 tion ; this mysterious result has a purely crystallographic 

 cause which completely escaped the notice of the original 

 investigators. Crystallographic examination of chloral 

 hydrate shows that when it solidifies from the molten state 

 it crystallises in long thin needles {/3) but during the follow- 

 ing ten days or so these needles slowly change into stout 

 rhomboidal plates (a) until finally the whole material con- 

 sists of plates formed at the expense of the original acicular 

 form ; the explanation of the variable heat of fusion is now 

 simple. The fi modification of chloral hydrate has a low 

 latent heat of fusion, only about one half that of the a form ; 

 during the prolonged and gradual conversion of the /3 into 

 the a modification the difference between the two heats of 

 fusion is as gradually liberated and dissipated, the result 

 being that to convert the a modification into liquid at the 

 same temperature requires the absorption of about twice 

 the quantity of heat necessary to liquefy the /3 modification 

 under similar conditions. 



Examples such as the above, indicating the important 

 influence of the crystalline form in complicating physico- 

 chemical problems relating to solid bodies might be multi- 

 plied indefinitely but all point the same moral, namely, that 

 since the crystalline form is practically always a complicat- 

 ing feature, it is an imperative necessity that all exact work 

 on solids, whether chemical or physical, should be accom- 

 panied by a careful study of the crystallographic questions 

 involved ; until the original investigator, more especially 

 the physicist, recognises and acts upon this conclusion, his 

 work on solids, though doubtless of much value in the work- 

 shop, will be comparatively unfruitful of results of scientific 

 interest. 



39 



