768 DETERMINATION OF THE BOILING POINT. 



700 mm , the boiling point is raised or lowered by nearly 

 0'0375 for each millimetre which the barometer stands 

 respectively above or below the normal pressure. For 

 example, if the barometer stands at 740, that is, 20 mm 

 below the normal pressure ; then the temperature of the 

 boiling point is 20 x '0375=0-75 below 100, and a 

 correct thermometer should indicate a temperature of 

 99-25. 



It follows, that in examining the correctness of the 

 boiling point on the scale of a thermometer the baro- 

 meter should be read at the same time, and the true 

 boiling point at the observed pressure should be cal- 

 culated as in the preceding example.' If the top of the 

 mercury becomes stationary at the temperature found 

 by the calculation, no error has been committed in 

 marking the boiling point, but if a difference is observed 

 it must be noted and applied as a correction to the 

 readings of the instrument. 



The aperture in the metal plate for supporting the flask used in 

 the determination of the boiling point should be just wide enough 

 to admit the lower part of the flask, up to nearly half its height ; 

 the ring of metal left should be at least 6 cm wide. It may be cut out 

 of a piece of sheet-iron or brass (zinc would melt) by drawing two 

 concentric circles on the metal, cutting along the outer circle with 

 the shears and with the chisel along the inner, and finally smoothing 

 the inner edge, so that the sides of the flask may be everywhere in 

 close contact with the metal and no projecting points may be left which 

 might injure the flask. If larger glass vessels are to be heated it is 

 advisable to place underneath them a piece of wire gauze, which is 

 bent so as to adapt it as nearly as possible to the shape of the lower 

 portion of the glass vessel. By means of such gauze the heat is 

 distributed more uniformly around the vessel than it would be with- 

 out it, and the cracking of the glass is prevented. Gauze of brass 

 wire is better for the purpose than iron gauze, which is much 

 sooner destroyed by the heat; brass gauze is also more pliable, 

 especially when it has been gently heated. For heating larger vessels 

 over a Bunsen burner it is best to purchase a special support, like 



