276 Mr. Morgan's Exferimenis to afiertam th^ 



times before the charge has made a fmall perforation in the; 

 bulb. If again a thermometer filled with mercury be inverted, 

 into a ciftern, and the air exhaufted in the manner I have de- 

 fcribed for making the experiment with the gage, a Torricellian, 

 vacuum will be produced; and now the electric light in the. 

 bulb, asw^ell as the fpark in the tube, will be of a vivid green; 

 but the bulb will not bear a frequent repetition of charges be- 

 fore it is perforated in like manner as when it has been ex- 

 haufted by an. air-pump. It can hardly be neceflary to obferve, 

 that in thefe cafes the ele6lric fluid affumes the appearance of a 

 fpark*, from the narrownefs of the paflage through which it 

 forces its way. If a tube, jj-O inches long, be fixed into a globe 

 8 or 9 inches in diameter, and the whole be exhaulled, the eledlric 

 fluid, after pafling in the form of a brilliant fpark throughout: 

 the length of the tube, will, when it gets into the inlide of the 

 globe, expand itfelf in all diredlions, entirely filling it wuth a 

 violet and purple light, and exhibiting a ftriking inftance of the 

 vafl elafticity of the eledric fluid., 



I cannot conclude this paper without acknowledging my obli- 

 gations to the ingenious Mr. Brook, of Norwich, who, by com- 

 municating to me his method of boiling mercury, has been the 

 chief caufe of my fuccefs in thefe experiments f. I have lately 



learned I 



* By cementing the firing of a guittar into one end oF a thermometer tube, a 

 f]}ark may be obtained as well as if the tube had been fealed hermetically. 



t Mr. Brook's method of making mercurial gages is nearly as follows. Let a 

 glafs tube L (fee fig. 2.)^ fealed hermetically atone end^ be bent into a right- 

 angle within two or three inches of the other end. At the diilance of about an inch 

 or lefs from the angle let a bulb (K), of about \ of an inch in diameter, be blown 

 ia the curved end, and let the reroainder of this part of the tube be drawn out (I) 



