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XXVII. On the Specific Gravity of certain Substances commonly considered lighter 

 than Water. By JOHN DAVY, M.D., F.R.S., L. and E., Inspector-General of 

 Army Hospitals, L.R. 



(Head, 3d April 1843.) 



THAT the common varieties of wood which float in water, owe their apparent 

 lightness to air contained in their structure, is generally admitted by those who 

 have paid any attention to the subject. By means of the air-pump, the fact is 

 clearly demonstrated. Under the exhausted receiver, after a certain time, the 

 time varying with the quality of wood, all the different specimens which I have 

 tried have sunk ; I may mention two or three in particular, as examples. A 

 piece of oak, weighing 29.7 grs., sank in distilled water, after having been sub- 

 jected to the air-pump three days ; a piece of deal, weighing 16.3 grs., similarly 

 acted on, floated ten days ; and a portion of the pith of the elder, weighing only 

 .133 grain, floated seven days. The temperature of the room in which the expe- 

 riments were made, was about 50 F. ; the air-pump was frequently worked in 

 the course of each day, and was in perfect order. After the exhaustion of the 

 air was carried as far as it well could be, the specific gravity of each wood, ex- 

 clusive of hygrometric moisture,* was found to be as follows : oak, 1.58 ; deal, 

 1.18 ; pith of the elder, 1.45.f 



The remark made on the common woods is applicable also to pumice and all 

 vesicular minerals, and admits of proof in the same manner. The specific gravity 

 of pumice is stated to be, according to BRISSON, .914 ; according to HOFFMAN, 

 .752 and .770 ; that is, in its ordinary condition, when its cavities are full of air. 

 But, when acted on by the air-pump, I find it is as high as 1.94. The subject of 

 the experiment was a portion of a specimen from Lipari, that weighed 31.8 grs., 



* The oak-wood lost by thorough drying, at a temperature a little below the scorching point (in- 

 cluding a small loss from the action of cold water), 18.3 per cent. ; the deal, 14.2 ; and the pith of the 

 elder, 13.3. 



t According to COUNT RUMFORD (Nicholson's Journal, vol. xxxiv., p. 322), the specific gravity of 

 oak is 15,344 ; of fir, 14,621, to water as 10,000. He arrived at these results, not by means of the air- 

 pump, but by the expulsion of air by boiling in water. The specific gravity of deal or fir-wood, as given 

 by him, is nearer the truth than that in the text, which is too low, for a reason which will afterwards be 

 assigned. I find, that when air is entirely, or nearly entirely expressed from it by compression in water, 

 that it sinks in a fluid of specific gravity 1.5 ; and that the pith of the elder, similarly treated, sinks in 

 the same fluid. 



The number 1.45, given in the text, as the specific gravity of the pith of the elder, was determined 

 hydrostatically, using a very delicate balance, affected by the one thousandth of a grain, when loaded with 

 500. 



VOL. XV. PART III. 5 M 



