Intelligence and Miscellaneous Articles. 485 



temperature so as to cut off the supply; but in a very short time 

 these two opposing forces come to an equilibrium, and the flame 

 scarcely varies in size. 



After trying the instrument in the form described, I experienced 

 a practical difficulty from the want of perfect contact between the 

 end of the tube C and the mercury, from which I found that it 

 might rise many degrees beyond the assigned limitwithoutsufl[iciently 

 lowering the flame ; this I at once saw might be overcome by making 

 the tube of a substance which would become ivetted by tiie mercury. 

 I tried a brass and also a copper tube, amalgamated at the end, but 

 they slowly dissolved in the mercury, which they rendered impure, 

 so that its rise and fall could not be depended on with the required 

 nicety. The substance now used is platinum, of which about half 

 an inch of the lower end of the tube is formed, amalgamated by 

 dipping it into a liquid amalgam of sodium and mercury. In con- 

 nexion with this I may mention, that platinum, iron and steel may 

 be readily amalgamated in the same way, or by using a strong so- 

 lution of caustic potash or soda in contact with the mercury. 



I have now used several of these instruments for various periods, 

 and have found them to answer well. In one operation, conducted 

 in Professor Gregory's laboratory (the fermentation of sugar into 

 butyric acid), one of them has been in use for upwards of six weeks, 

 keeping about five gallons of liquid at a temperature of 98° F., and 

 it has not been observed to vary. I have also another in an appa- 

 ratus for artificial incubation, where the temperature in the well 

 part is kept at 120° F. For this latter purpose I consider it well- 

 adapted, as it may be placed in the vessel along with the eggs, and 

 thus the use of hot water be altogether dispensed with, as well as 

 the constant attendance required to regulate the heat applied to the 

 eggs. 



I need not take up space with pointing out many applications 

 that may be made of the instrument ; these will occur to every one : 

 but there is one I may allude to, viz. obtaining products of the de- 

 composition of organic bodies at fixed temperatures, which has 

 hitherto been somewhat difficult, as different substances are formed 

 as the temperature varies to which they are subjected. The only 

 lijnit to its application is the same as in the ordinary thermometer, 

 viz. the boiling of the mercury ; this might be overcome by using 

 an easily fusible metal, such as tin, and constructing the instrument 

 of iron, when its form might also be modified to suit particular 

 circumstances. — From the Chemical Gazette for May 15, 1850. 



ON SOME PHjENOMENA OF DEFECTIVE VISION. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, Brompton, April 6, 1850. 



One of the earliest evidences of old age creeping on is experi- 

 enced in defective vision ; and some months ago my attention 

 was directed to a peculiarity in the phaenomena attending it which I 

 do not recollect to have seen noticed in books. It may possibly be 

 peculiar to my own eyes : but I think not ; for though there is a 



