702 PHILOSOPHICAL TRANSACTIONS. [ANNO 1785. 



rowness of the passage through which it forces its way. If a tube, 40 inches 

 long, be fixed into a globe 8 or Q inches in diameter, and the whole be exhausted, 

 the electric fluid, after passing in the form of a brilliant spark throughout the 

 length of the tube, will, when it gets into the inside of the globe, expand itself 

 in all directions, entirely filling it with a violet and purple light, and exhibiting a 

 striking instance of the vast elasticity of the electric fluid. 



I cannot conclude this paper without acknowledging my obligations to the 

 ingenious Mr. Brook, of Norwich, who, by communicating to me his method of 

 boiling mercury, has been the chief cause of my success in these experiments.* 

 I have lately learned from him, that he has also ascertained the non-conducting 

 power of a perfect vacuum ; but what steps he took for that purpose I know not. 

 Of his accuracy however I am so well convinced, that had 1 never made an 

 experiment myself, I should, on his testimony alone, have been equally assured 

 of the fact. To most of the preceding experiments Dr. Price, Mr. Lane, and 

 some others of my friends, have been eye-witnesses, and I believe that they were 

 as thoroughly satisfied as myself with the results of them. I must beg leave to 

 observe to those who wish to repeat them, that the first experiment requires 

 some nicety, and no inconsiderable degree of labour and patience. I have boiled 

 many gages for several hours together without success, and was for some time 

 disposed to believe the contrary of what I am now convinced to be the truth. 

 Indeed, if we reason a priori, I think we cannot suppose a perfect vacuum to be 

 a perfect conductor without supposing an absurdity : for if this were the case, 



* Mr. Brook's method of making mercurial gages is nearly as follows. Let a glass tube l (see 

 fig. 12, pi. 9), sealed hermetically at one end, be bent into a right angle within 2 or 3 inches of the 

 other end. At the distance of about an inch or less from the angle let a bulb k, of about ^ of an 

 inch in diameter be blown in the curved end, and let the remainder of this part of the tube be drawn 

 out at I, so as to be sufficiently long to take hold of, when the mercury is boiling. The bulb k is 

 designed as a receptacle for the mercury to prevent its boiling over, and the bent figure of the rube 

 is adapted for its inversion into the cistern ; for by breaking off the tube at m, within ^ or 4 of an 

 inch of the angle, the open end of the gage may be held perpendicular to the horizon when it is 

 dipped into the mercury in the cistern, without obliging us to bring our finger, or any other sub- 

 tance, into contact with the mercury in the gage, which never fails to render the instrument 

 imperfect. It is necessary to observe, that if the tube be 14 or 15 inches long, I have never been 

 able to boil it effectually for the experiments mentioned in this paper in less than 3 or 4 hours, though 

 Mr. Brook seems to prescribe a much shorter time for the purpose ; nor will it even then succeed, 

 unless the greatest attention be paid that no bubbles of air lurk behind, which to my no small morti- 

 fication I have often found to have been the case ; but experience has at length taught me to guard 

 pretty well against this disappointment, particularly by taking care that the tube be completely dry 

 before the mercury is put into it ; for if this caution be not observed, the instrument can never be 

 made perfect. There is however one evil which I have not yet been able to remedy ; and that is, 

 the introduction of air into the gage, owing to the unboiled mercury in the cistern ; for when the 

 gage has been a few times exhausted, the mercury which originally tilled it becomes mixed with 

 that into which it is inverted, and in consequence the vacuum is rendered less and less perfect, till at 

 last the instrument is entirely spoiled. — Orig. 



