I 
ON THE DECOMPOSING POWER OF HOT STEAM. 185 
decomposing power of water at high temperatures when 
brought in contact with certain chemical compounds, espe- 
cially the oxy and haloid salts, of the alkalies and alkaline 
earths. It has long been known that a partial decomposi- 
tion is effected in certain salts which have a strong tendency 
to retain their water of crystallization when they are dried 
rapidly with a strong heat — as the chlorides of magnesium 
and calcium; and this apparently spontaneous decomposi- 
tion appears to be in proportion to the intensity of the tem- 
perature, to which the hydrous salt can be brought, before 
the water is volatilized. Mr. Tighlman, in reflecting on this 
phenomenon, conceived the idea that it was the nascent 
vapour which acted on the residual anhydrous salt which 
it enveloped, and that the same changes might be effected 
by bringing aqueous vapour, generated in a boiler, and 
heated by passing through hot tubes, into contact with the 
anhydrous salts in a suitably arranged furnace. In testing 
his idea practically, Mr. T. found his anticipations fully 
realized for " not only the anhydrous chloride of calcium,'^ 
but the chlorides of strontium, and barium, could be rapidly 
decomposed by exposing them at a high red heat, to a cur- 
rent of steam ; hydrochloric acid was copiously evolved 
and escaped with the excess of steam, whilst the bases of 
the respective salts were left in a free state. ^' 
The facility of the decomposition, other circumstances 
being equal, appears to be in ratio to the volatility of the 
acid and fixedness of the base ; hence chlorides are more 
easily decomposed than sulphates. 
The oxy-salts do not, as in the haloid compounds, require 
oxygen and hydrogen to give basic and acid character to 
their elements before submitting to decomposition. Yet, 
nevertheless, it is well known that the sulphates of magne- 
sia, lime, strontia and baryta sustain the strongest heats, j»er 
56, without decomposing ; but when a current of hot steam 
is brought in contact with them in a heated state, decompo- 
sition ensues, their bases remain fixed in a free slate, anhy- 
17 
