76 The Rev. Dr. Callan's Experiments on the ■ 



used, I have put a strip of vulcanized India-rubber across the 

 hole through which the gases enter into each of the iron vessels. 

 The strip of vulcanized India-rubber acts as a valve, vi^hich 

 opens inwards and admits the gases into the vessel ; and when 

 pressed outwards by the expansive force of the exploded gases, 

 it closes the hole and prevents the escape of any part of the water 

 into the gas bag or voltameter. Hence it is evident that in our 

 apparatus dangerous explosions cannot happen, either because 

 the water may be gradually ejected from the iron vessel through 

 the jet or back into the gas bag, or because the flame might 

 ascend through a continued series of bubbles of the mixed gases. 

 The size of the iron vessels should be in proportion to the quan- 

 tities of mixed gases inflamed, and to the length of time the 

 flame is kept up. Should a person wish to continue the lime 

 light for a very long time, he would do well to have two pairs of 

 vessels and change them every hour or half-hour, or examine 

 occasionally whether the water was carried away by the stream 

 of the gases. I have several times tried the large vessel alone 

 without any accident, although the gases above the water occa- 

 sionally exploded when they were not pressed through the jet 

 with suQicient force. I believe that this vessel )nay be used 

 alone without danger ; but I would recommend in all eases the 

 use of two vessels, one of which should be a good deal larger 

 than the other, tliat the small one be about 3 inches in dia- 

 meter and 6 inches high, and that the experimenter examine 

 occasionally whether they contain water. I always kept the gas 

 bag in a place in which, though an explosion should occur, no 

 injury could be done to any person. On one occasion I filled 

 the small iron vessel with shot instead of M^ater. The shot pre- 

 vented the return of the flame for a little time, but after five or 

 six minutes the gases in the small vessel and in the upper part 

 of the large one exploded, but the flame did not descend through 

 the water. The mixed gases which I employed were always 

 obtained by the decomposition of water. One of the gas bags 

 commonly used for the gas microscope and polariscope may be 

 filled in two or three hours by a cast-iron battery of six cells and 

 as many zinc plates, each 6 inches square. I always employed 

 nitrosulphuric acid which had been previously used. Acid 

 which had been used before answers very well for producing 

 decomposition ; because, for this effect, the cells of the battery 

 must be so arranged that its intensity will not exceed that of 

 three, or at most four cells in series. When the gases are ob- 

 tained by the decomposition of water, they are always mixed in 

 the proportions in which they answer best for the production of 

 intense heat and light; hence, when it can be done without 

 dangei', it is better to produce the lime light by the decompo- 



