284 EIGHTH REPORT — 1838. 



men of cast iron whose specific gravity was required was filed 

 accurately by a steel gauge to a cube of 0*75 of an inch; this 

 cube was weighed in air at a temperature of GO^Tahr. A small 

 glass cylinder Avas provided capable of being closed air tight at 

 its open end by a circular disk of thin Bohemian plate glass, 

 equal in diameter to its exterior. The size of the cylinder was 

 such as just to admit, without contact at the angles, a cube of. 

 the above size, and its weight with the plate-glass cover, was 

 under 100 grains when empty. Its weight was then accurately 

 determined from the mean of a number of weighings, when 

 quite full of distilled water, free from air, at 60° Fahr. The 

 filling is easily accomplished by pouring in the water after 

 having been boiled in vacuo and cooled, until its surface rose a 

 little above the edge or top of the cylinder, and then sliding on 

 the glass plate. A number of the iron cubes having now been 

 weighed in air were thrown into a considerable volume of di- 

 stilled water at 60°, and placed under the exhausted receiver 

 of the air pump, and agitated until all air bubbles had escaped. 

 The glass cylinder being filled as before, each cube was taken 

 out of the water by forceps and placed in the cylinder, from 

 which it of course expelled its own bulk of water ; the cylinder 

 was now closed, dried rapidly in bibulous paper with gloved 

 hands, and weighed, the temperature of the apartment being 

 preserved carefully at 60° Fahr. 



It is sufficiently obvious from these data that we get the spe- 

 cific gravity from the formula 



s = ^, 



W 



— where S is the specific gravity of cast iron, tt; = the weight of 

 a cube of distilled water = 0'75 of an inch, s = the specific gra- 

 vity of water, and W the weight of the cube of cast iron, 

 equal in volume to the cube of water. 



This method possesses several advantages in rapidity and ease 

 of execution, and in precision of result, besides involving a check 

 upon any serious error of experiment in every instance ; for as 

 each cube is weighed in air, and the weight of a cubic inch of 

 distilled water at 60° Fahr. is well determined, the specific gra- 

 vity of each ciibe is at once known within the limit of error in 

 the gauged dimensions to which the cast iron cube is filed. 

 There was found to be no difficulty in drying the outside of the 

 cylinder, so that it did not change its weight in the balance, i. e. 

 perfectly, and no sensible evaporation took place from under 

 the plate-glass disk, after remaining in the balance for forty- 

 eight hours. 



