Polytechnic Association. 707 



be constructed to test this ; and I think it would be found that the 

 two ends would fly in opposite directions, as they do in boiler explo- 

 sions. I have no method to propose to prevent boilers from explod- 

 ing. It may be that we are making money by keeping at the very 

 borders of danger. "We can secure perfect immunity from danger, if 

 we choose. We can make the shell so thick that it cannot give way ; 

 but we do not consider it expedient to do so. If we kept entirely 

 clear from the borders of danger, we should never know where we 

 were. We need something occasionally to warn us of the profitable 

 line to pursue. 



Mr. Hudson stated that while filling a soda fountain, it had 

 exploded, the two pieces flying in opposite directions, although it was 

 evident that the cause was over-pressure and not electricity. 



Mr. Norman Wiard — It is unnecessary to inquire where the extra- 

 ordinary force comes from, for in an ordinary boiler there is force 

 enough stored up to raise 350,000 lbs. 750 feet high, which is a larger 

 amount than has ever manifested itself in an explosion. There were 

 some experiments in the Palace Garden a few years ago, in which 

 electricity was derived from a boiler ; but the boiler was insulated ; 

 and a spark could not be obtained from a boiler which was not. I 

 do not, therefore, think it necessary to discuss the question of elec- 

 tricity. But I wish to say that Mr. Blan chard is mistaken in sup- 

 posing that boilers can be made strong enough by using thicker iron. 

 A shell an inch in thickness would not be so strong as a shell of one- 

 fourth that thickness ; for the reason that the unequal expansion of 

 such thick plates would cause a rupture in a short time. 



Dr. L. Bradley — I cannot assent to the proposition in this paper, 

 that thunder is due to the explosion of gases that have been decom- 

 posed by electricity ; the quantity of gas evolved is so small compared 

 with the atmosphere around it, and it requires so small an admixture 

 of common air to make the gases non-explosive. 



The President — There may be an explosion without any increase 

 of volume. For instance, mix hydrogen and chlorine in the dark. 

 Expose them to light, and they will combine chemically with an 

 explosion, although the volume is unchanged. Choosing partners for 

 a dance causes a considerable stir in a ball-room, although the persons 

 present take up no more room than before. A chemical selection of 

 partners, by particles of matter, may be sudden and violent. 



Mr. F. A. Woodson — It is always improper to look for the cause 

 of a phenomenon at a distance, when there is adequate cause close at 

 hand. In the relations of steam, water, metal and fire, we have 



