28 ON THE HEAT EVOLVED 



specific gravity of any wood is correctly ascertained, its absolute 

 weight may be determined thereby. 



The usual method of ascertaining the specific gravity of 

 wood, as laid down in the books, is manifestly incorrect, as the 

 absorption of water, during its immersion, produces an enlarge- 

 ment in the magnitude of the body, not compensated for by 

 adding to the water weight, if the body is lighter, (or deduct- 

 ing, if heavier than water,) the weight of water found to have 

 been absorbed, and this absorption must constitute complete 

 saturation before the water weight can be accurately ascertained, 

 because, during this process of absorption, the air being con- 

 stantly expelled from the body, part of it adheres to it in small 

 globules, and renders it more buoyant, in proportion as this bulk 

 of air is lighter than the same bulk of the body ; consequently, 

 the body weighs less than it should do, and this cause of error 

 cannot be counteracted by an attempt to weigh the body "expe- 

 ditiously" as is recommended. During this necessary process of 

 saturating the body with water, the wood increases in magni- 

 tude, and its specific gravity will be found less than it should 

 be ; and the difference will be seen to be very considerable, 

 when it is stated that the specific gravity of a piece of dry 

 wood, weighing in air 11.15 grains, was, by the common me- 

 thod found to be .556, and the same piece of wood being then 

 dried with great care to its former weight, its specific gravity 

 found by a process free from this objection, (hereafter to be 

 described,) was .619, the difference in which would be 282 lbs. 

 in one cord of wood. 



The specific gravity of those bodies which do not change 

 in their magnitude by the absorption of water, and which have 

 no fissures, may be correctly obtained by the common method, 

 as the water absorbed is retained in the body, and can thereby 

 be ascertained, as it will be of the exact weight by which the 

 water weight had been increased or diminished in consequence 

 of the expulsion of an equal bulk of air from the body. 



Onr object in ascertaining the specific gravities of bodies, 

 is to find the proportion of their weights under the same 

 volume. Now, by the volume of a body, is to be understwd 



