Ac/ricultural Steam-boilers. 
429 
Mr. William Crook, editor of the ' Chemical Gazette,' who has had 
a good deal to do with, the new method of analysing metals by light, 
in a letter to me, says : — 
" There are many subjects connected with the ebullition of water which are 
not generally known, but which would throw considerable light on many boiler 
explosions. Any one who has watched perfectly pure water boiling in a clean 
glass vessel, open at the top, will have observed the tremendous force with 
wliich the steam bursts forth at intervals, whilst at intermediate times tlie 
liquid is quite at rest. I have sometimes had thin glass flasks shattered to 
pieces by this explosive force of the boiling water, and that under the ordinary 
atmospheric pressure. The presence of different chemicals dissolved in the 
water has considerable influence on this percussiv6 ebullition ; alkalies, for 
instance, increasing the violence ; whilst if a gas is being evolved in the liquid, 
the boiling takes place with perfect tranquillity. A great deal, therefore, 
depends upon the quality of the water and the mineral impurities which it 
contains, some waters being quite free from this property, and others possessing 
it to a dangerous extent. Much also depends upon the amount of insoluble 
matter (carbonate or sulphate of lime) deposited in the boiler, and upon tlie 
state of aggregation in which the deposit is formed — a sandy deposit being of 
little importance, but a hard stony cake being very liable to give rise to inju- 
rious results." 
The boilers of ^ixecZ engines have an advantage over those ol portahlc 
engines, in that they are constantly supplied with the same kind of 
water and the same quality of coal, or nearly so, and in their being 
constantly under the care of the same individual, who, as in the 
di'essing of a horse, can work better with an engine in " its o^^^l 
stall," so to speak, than he could with one going about to dilferent 
parts of the country. But iwrtahlc engines are differently circum- 
stanced, especially when in the hands of persons who have hired them ; 
and their boilers are filled one day out of one ditch, and the next out 
of another ; one day with soft water, another day with hard water ; iu 
short, with water of different degi-ees of impiu'ity. At the same time 
they are heated with different kinds of fuel ; they go through a largo 
amoimt of weather-wear ; they are too often imperfectly cleaned ; and 
from their being hurried fi'om one job to another, they are so fre- 
quently neglected as to be additionally liable to accident. Nor is 
this an unimj^ortant circumstance, for, according to Mr. Crook, as 
already quoted, a gi'eat deal depends upon the quality of the water and 
the mineral impurities it contains, and a great deal on the state of 
aggregation in which the deposit is formed. 
Water when it boils, has in it a certain quantity of air. If that air 
be boiled out of it, the boiling is checked. It is a mistaken idea that 
water boils always at the temperature of 212° ; for, according to the 
degree of pressui-e, it may either boil at a lower temperature, or be 
made not to boil, but remain quiescent, at a temperature far higher 
than 212°. The following extract from a work by Dr. Carpenter 
shows in a few words how, under certain pressure, water is made to 
boil, and how vapour is formed : — • 
" Water without air boils only at intervals, and stops altogether ; but if, 
when it has stopped, air be admitted by means of any solid substance which is 
put into it, however small the quantity of that substance, it will begin boiling; 
