6 History of physical Science from [July, 



hydrogen gas is evolved, and this continues till the whole of the 

 •zinc is dissolved. — (Ann. de Chim. et Phys. v. 301.) 



6. Metallic Thermometer. — One of the most beautiful and 

 convenient little instruments for which we are indebted to the 

 modern improvements in chemistry, is a thermometer of metal 

 contrived by M. Breguet. It consists of slips of two metals 

 unequally expanded by heat twisted into a spiral : to the extre- 

 mity of the spiral is fixed an index, which moves round a 

 graduated circle pointing out the temperature. It is obvious 

 that when the- spiral is heated, the index will move in one 

 direction, and in another when the spiral is cooled, because it 

 will twist or untwist itself according to the changes of temper- 

 ature to which it is subjected. The two metals employed are 

 silver and platinum ; and in order to render the extreme points 

 more fixed, and to prevent sudden starts, a slip of gold, the 

 expansibility of which is intermediate between that of silver and 

 platinum, is soldered between these two metals. This thermo- 

 meter is more delicate than any mercurial thermometer what- 

 ever. It is even more delicate than an air thermometer. This 

 spiral thermometer and a mercurial one were placed together 

 under the receiver of an air-pump. The temperature at the time 

 of the experiment was 66-2 . The mercurial thermometer when 

 the air was pumped out sunk 3*6° ; but the spiral thermometer 

 fell 41-4°, or it fell down to 24-8° Fahrenheit.— (Ann. de Chim. 

 et Phys. v. 312.) 



II. ATOMIC THEORY. 



Accurate chemical experimenting can scarcely be dated 

 further back than the introduction of the atomic theory into 

 chemistry. So short a space of time has elapsed since that 

 introduction, and the number of substances to be examined is so 

 great, that it is rather surprising that so considerable progress 

 has been made in the determination of the weight of the atoms 

 of bodies. By far the greatest share of this Herculean task has 

 fallen to the lot of Berzelius ; and whether we consider the 

 accuracy or the indefatigable activity of this incomparable 

 chemist, it could not have fallen into better hands. 



From the extreme difficulty of performing chemical experi- 

 ments with perfect accuracy, it becomes an object of great 

 importance to repeat them very often. The only way by which 

 minute accuracy can be introduced into chemical numbers, is 

 by following the plan which has been so long customary with 

 astronomers. They make a great number of observations with 

 all the requisite care ; and by taking the mean of the whole, the 

 error is reduced to a very small quantity ; and it obviously 

 diminishes as the number of observations, the mean of which is 

 taken, increases. Were the same method followed by chemical 

 experimenters, their errors would diminish in the same way, and 



