found in the Boilers of Steam, Engines. 841 
into low wines, to throw a quantity of soap into the still every 
time it is charged or filledj w^hich has the effect of causing the 
steam to rise more quickly, and more disengaged from the residu- 
ary matter of the process. In this instance the soap acts chemi- 
cally, by uniting with the water and residuary matter. We do 
not, however, see any analogy between this process and the 
effect produced in the engine-boiler before mentioned. 
The sediment in boilers produced after wet weather, is chief- 
ly composed of clay, and does comparatively little injury to the 
boiler ; but in general the common mine water which percolates 
very slowly through the strata, produces a sediment of sulphate 
of lime, which adheres so closely to the bottom of the boiler 
that it cannot be removed, but by picking it off with a sharp 
iron instrument ; and this sediment, when removed, has fre- 
quently a thin scale of the iron-plate of the boiler adhering to 
it. In this way the boiler is not only injured, but if the sedi- 
ment accumulates at any part of the bottom, the plates are lia^ 
ble to become red hot at that place, which greatly injures them. 
To lessen these injurious effects, it is the practice to throw 
into the boiler a quantity of peat-earthy in its natural plastic 
state, which is found to have a considerable effect in preventing 
the sediment from adhering so closely to the boiler plates. 
Alloa, February 8. 1820. 
Art. XXV . — On the Absorption of Polarised Light by Doubly 
Refracting Crystals *. By David Brewster, LL. D. 
F. R. S. Lond. and Edin. 
T. HERE are no phenomena in optics which are less under- 
stood than those which relate to the Absorption of lights or that 
property of transparent bodies by which they detain and assi- 
milate to their own substance a portion of the light which pe- 
netrates them. It became, therefore, a matter of great import- 
ance to discover in regular crystals some analogous phenome^ 
na, where the absorptive power acted in relation to certain fixed 
lines in their crystalline form, as it was only by such a disco- 
* Abridged from the Philosojihical Transactions for 1819, page 11, 
