438 Abstract Report of Aj/icuUural Discussions. 
of limo (carbonate and sulphate of lime) on boiling and concentration of tlie 
licjuid in the boiler, instead of gradually separating and collecting into a hard 
crystalline mass, separate in more powdery particles, which attach them- 
selves to the sawdust or the spent tan, which, moreover, rubbing against all 
sides of the boiler prevent in a purely mechanical way the deposition of a 
stone-lii-;e incrustation. From time to time the water left in the boiler must 
be run off, and some fresh sawdust or Ian be introduced into it. The light 
particles of savvdust or tan have another beneficial efi'ect, to which attention 
was not directed iu the discussion at Hanover Square. It is this : these 
particles, being lighter than water, float on the surface while it is cold ; when 
it boils, they are thoroughly distributed in all parts of the water in the boiler, 
which has the effect of causing the steam to be given off regularly, without 
producing sudden jerks. This is a great advantage, for there cannot be a 
doubt that the more regularly and easily steam is produced, the less chance 
there is of a boiler explosion — of course, other cii'cumstances being equal. 
The mechanical agent in this case acts like air, causing steam to be more 
easily given off. 
*' 2. The deposition of hard crystalline boiler-deposits may be prevented by 
chemical means. Caustic soda, in my opinion, is by far the best thing that 
can be used for that purpose. In hard water we have suliDhate and bi- 
carbonate of lime — sometimes more sulphate tlian bicarbonate, and sometimes 
more of the latter than the former. Caustic soda removes both impurities. 
" If bicarbonate of lime only is present, the caustic soda takes one-half of its 
carbonic acid, and becomes carbonate of soda — a soluble salt which remains in 
solution ; and the bicarbonate of lime (a soluble salt) loses half its carbonic 
acid, and becomes changed into insoluble or neutral carbonate of lime, wliich 
is deposited. Professor Wilson, who explained so well the effect of lime-water 
on bard waters, I have no doubt will notice the analogy between the action 
of caustic soda and caustic lime — for lime-water is mtjrely a solution of caustic 
or quick-lime in water. The principle of their action is precisely the same. 
In one case we have in solution — 
Bi-carbonate of soda, Ca 0, 2 C 0^ 1 equivalent of lime -f- 2 of carbonic acid. 
or Ca, C 0, 
+ 
C Oj, and if we add 
Ca 0 = 1 equivalent of caustic lime, 
we obtain 
Ca 0, C 0„ 
or insoluble carbonate of lime. 
-f Ca 0, C 0.! insol. carb. of hme. 
In the other case to 
Ca 0, C Oj 
C Oj i. e. bicarb, of lime, 
we add 0, i. e. caustic soda, 
and obtain 
Ca 0, C O2 e. insol. or neutral -F Ka 0, C O2 i. e. soluble carl onate 
carb. of lime, of soda. 
" If the water, as is generally the case, contains sulphate as well as carbonate 
of lime, the sulphate is likewise deposited .by the addition of caustic soda. 
Iu the first jilnce we obtain, as shown just now, insoluble carbonate of lime 
and soluble carbonate of soda ; but in the next place the carbonate of soda 
and suljjhate of lime, by mutual exchange of their acids, become carbonate of 
lime (which lalls down) and suljjhatc of soda (which remains iu solution) : — 
