VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. Qgg 



water of the temperature of 62°, and it loses in it 13.333 grains, if the same 

 piece of iron be weighed in water of the temperature of 75°, it will lose but 

 13.313 grains; for the losses of weight will be as the weights of equal volumes 

 of water at those temperatures, which we have seen are as 253.18 to 252.8 ; 

 therefore its specific gravity in water of the temperature of 62° will be 7-4Q ; and 

 in water of the temperature of 75°, 7-51 1 ; but we may correct this by the above 

 analogy, for 253.8 : 252.18 :: 7.511 : 7-49. By this means we obtain the advan- 

 tage of discovering the true weight of a cubic foot of any substance whose spe- 

 cific gravity is known, which it is now plain cannot be known when bodies are 

 hydrostatically weighed at any temperature a few degrees above or below 02°, with- 

 out such reduction, or subtracting the quantities in the 4th column. 



This method is equally applicable, and with equal necessity, to other means of 

 finding specific gravities, as areometers, the comparison of the weights of equal 

 measures of liquids, the different losses of weight of the same solid, when 

 weighed in different liquids, &c. In all which cases the weight of water at 

 62°, or the loss of weight of a solid in water at 62°, should be found by the 

 above analogy. Dr. Hales and some others have estimated the weight of a cubic 

 inch of water at 254 grains, which is an evident mistake, as it is true in no degree 

 of temperature, and produces an error of more than 3 ounces in the cubic foot. 



XIV. Electrical Experiments made to ascertain the Non-conducting Power of a 

 Perfect Vacuum, &c. By Mr. JVm. Morgan, p. 272. 



The non-conducting power of a perfect vacuum is a fact in electricity which 

 has been much controverted among philosophers. The experiments made by 

 Mr. Walsh, f. r. s. in the double barometer tube, clearly demonstrate.! the im- 

 permeability of the electric light through a vacuum ; nor was it, I think, pre- 

 cipitate to conclude from them the impermeability of the electric fluid itself. But 

 this conclusion has not been universally admitted, and the following experiments 

 were made with the view of determining its truth or fallacy. When I first at- 

 tended to the subject, I was not aware that any other attempts had been made 

 besides those of Mr. Walsh ; and though I have since found myself to have 

 been in part anticipated in one of my experiments, it may not perhaps be im- 

 proper to give some account of them, not only as they are an additional testi- 

 mony in support of this fact, but as they led to the observation of some phe- 

 nomena which appear to be new and interesting. 



A mercurial gage b (fig. 11, pi. 9,) about 15 inches long, carefully and accu- 

 rately boiled till every particle of air was expelled from the inside, was coated 

 with tin-foil 5 inches down from its sealed end a, and being inverted into mer- 

 cury through a perforation d in the brass cap e which covered the mouth of the 

 cistern h ; the whole was cemented together, and the air was exhausted from the 



4 u 2 



