Description of a Metallic Thermometer. 1 19 



If the above mode of determination at all approach to 

 correctness, it would appear that the specific gravity of 

 rock salt is diminished, by being broken into small frag- 

 ments, from 2125 to 2112, probably in consequence of the 

 quantity of air which the fragments envelop, and which 

 cannot be entirely separated by agitation. From the num- 

 bers given in the last column, it is evident that the smaller- 

 grained salts are specifically heavier than those which are 

 composed of larger and more perfect crystals. A difference 

 of only one or two hundredth parts of a cubic inch is 

 perhaps entitled, in a process of this kind, to little reliance; 

 and I do not therefore regard it as indicating any material 

 difference in the specific gravity of the first four or last 

 three salts submitted to experiment. But when the dif- 

 ference amounts to eight hundredths, as between the small- 

 and large-grained salt, it may safely be imputed to an in- 

 ferior specific gravity in that species, which occupies so 

 much greater a proportional bulk*. 



The last series of experiments proves decisively, that in 

 an important quality, (viz. that of specific gravity,) which 

 is probably connected with the mechanical property of 

 hardness and compactness of crystals, little or no difference 

 is discoverable between the large-grained salt of British, 

 and that of foreign manufacture. If ho superiority, then, 

 be claimed for British salt as applicable to ceconomical 

 purposes, on account of the greater degree of chemical pu- 

 rity which unquestionably belongs to it, it may«safely, I 

 believe, be asserted that the larger-grained varieties are, as 

 to their mechanical properties, fully equal to the foreign 

 bay salt. And the period, it may be hoped, is not far di- 

 stant, when a prejudice (for such, from the result of this 

 investigation, it appears to be,) will be done awav, which 

 has long proved injurious to the interests and prosperity of 

 an important branch of British manufacture. 



[To be continued.] 



XIX. Description of a Metallic Thermometer for indicating 

 the higher Degrees of Temperature, 



To Mr. Tilloch. 



Sir, 1 beg leave, through the medium of your Magazine, 

 briefly to mention the principle of a new thermometer, 



* M. Hassenfratz seems to have suspected that a difference in the specific 

 gravity of the same salt may be occasioned by a variation in its state of 

 crystallization. De la Pesanieur specify ue des Sels t Ann. de Chim. xxviii. p. I7 r 



H 4 con- 



