In trying to reduce the inexplosibility to the utmost, Messrs. Serpollet Brothers have succeeded in constructing a type of boilers which may be called _absolutely inexplosible_, and this result has been obtained by reducing the capacity of the boiler to practically _nil_, thus rendering the explosibility also _nil_, for under the circumstances the relation between capacity and vaporizing power becomes itself _nil_.
[Illustration: FIG. 1.--INSTANTANEOUS VAPORIZATION BOILER OF MESSRS. SERPOLLET.
1. General view of boiler (experimental arrangement). 2. Cross section of boiler (natural size). The line A B indicates, at somewhat exaggerated scale, the cross section of the interior empty space of the boiler.]
The method employed for this purpose by Messrs. Serpollet is an extremely simple one. A cylindrical steel tube of convenient diameter and sufficient thickness is rolled flat at a temperature below the white heat of the metal, and the last touch of the rollers is given to it when already cold. By this means a flat tube is obtained, the empty interior space of which looks in a cross section (Fig. 1, No. 2) like a black line not thicker than a hair, and measures from 0.1 to 0.3 millimeter. This tube is finally rolled up in the form of a spiral, or left straight, according to the use to be made of it, and put into an appropriate furnace (Fig. 1, No. 1). To either end of the tube a joint is attached, the one for the purpose of admitting the water, the other for admitting the steam.
When under these circumstances the tube has been heated to a high temperature in a convenient fire box, the water which has been pumped into it, by a feed pump fastened to one of its extremities, is instantly changed into steam and escapes at the other end at a pressure and in a state of dryness depending on the working conditions of the apparatus. The ingenious and really original and novel idea in this invention is this flattened tube, which constitutes an actual capillary boiler inside of which the water squeezed in between its walls cannot assume its spheroidal state, and the formation of drops becomes absolutely impossible. There exists no longer a residue of hot water, nor are water gauges, safety valves, or any other of those numerous accessories required which make all steam boilers so complicated and which augment considerably their cost.