CRITICAL NOTICES OF NEW PUBLICATIONS. 325 



that the foam being thrown upon the hot iron, is instantly vaporized. 

 If this be true there are cases in which the opening of a safety valve 

 may be a source of danger instead of one of safety ! And this result 

 was actually found experimentally to be true by M. M. Tabareau 

 and Rey, of Lyons. But the fact of this foaming is also interesting 

 as affecting the indications of the gauge-cocks and floats, commonly 

 used to show the level of the water in a boiler. It must be especially 

 effective in a small high pressure boiler. 



The Committee found, by experiment, that when water boiling 

 under pressure is relieved from that pressure a foaming commences, 

 near the point at which the reUef is given, extending throughout the 

 fluid. That this is greater as the opening made is greater, the relief 

 more sudden, and the previous pressure greater. In one of the ex- 

 periments indications of water were found by a guage-cock two inches 

 above the true level of the water in the small experimental boiler, 

 these guage-cocks being at the time open. 



The glass guage-tube used in our locomotive engines they found 

 not to be affected by this foaming, until it reached the top of the 

 tube. They recommend its use strongly, and propose to substitute 

 green glass in high pressure boilers, for the white glass which erodes 

 under the action of high steam. 



It is obviously impossible to determine, as a general phenomenon, 

 whether the steam produced by the projection of foam upon the 

 heated sides of the boiler, produces more steam than that which 

 escapes through the opening causing the foaming. It depends upon 

 circumstances liable to vary in every case. This view the Com- 

 mittee take of the subject. They refer to the experiments of M. 

 Arago, at Paris, made upon boilers not unduly heated, in which the 

 mercury guage always fell on making an opening from the boiler ; to 

 their own experiments on a boiler of which the sides were heated 

 when the same result followed ; and to those of M. M. Tabareau and 

 Rey, when the boiler was surrounded by a charcoal fire, and when 

 the reverse always took place, the pressure being increased by making 

 an opening. It must be admitted then, that with a boiler presenting 

 a great extent of highly heated surface upon which foam may be 

 thrown, the making of an opening may be attended with danger. 

 Indeed the successive explosions, recorded by Mr. John Taylor to 

 have occurred in connected boilers in the Polgooth mines, do not ad- 

 mit a contrary supposition, for the second boiler had just had an 

 opening of large size made by the explosion of the first, with which 

 it had been in connection. . 



To avoid the source of danger resulting from the presence of 

 heated metal, it is necessary to have some means of ascertaining the 

 temperature of those parts of a boiler which are most liable to be- 

 come overheated, so as to give notice before they reach a temperature 

 at which danger would result. The Committee discuss various in- 

 ventions which have been brought before them for this purpose, and 

 give the experiments made upon one proposed by their chairman. 



