SCIENTIFIC SUMMARY. 
229 
from decomposing’ feldspars. But one of tlie most striking facts in my 
examinations occurred at our last analysis of a flame from a re-heating 
furnace on the Lehigh, at the wire works of Stuart and Co. The workmen 
held partly out a bar of intensely heated iron on the hearth of the furnace, 
when, at rapid intervals, the dark lines which are seen in the solar spectrum 
appeared faintly, hut certainly flitting over the spectrum of the fierce flame 
by which the intensely heated iron was enveloped. An instrument, of a 
circular form, is in course of construction, under my direction, for the easy 
examination of these flames, and which may be used at any time and at 
considerable distances ; and I am hoping that such shall he its sensitiveness, 
that the furnace master may sit in his room and know much of the efficiency 
and value of the operations proceeding at the furnace by its use. I am 
situated on a hill, and by means of my instrument, placed upon my dinner 
table, I can get a beautiful spectrum from a re-heating furnace situated not 
much less than a half-mile from my instrument, and am able to detect the 
sodium in the coal, or from the decomposed fire brick, and also any lime, 
potash, &c., which proceeds from the furnace mouth. I have no doubt that 
some exceedingly important uses may be made of this discovery of the 
spectroscope in the line of metallurgical operations.” — ^Vide Chemical Neius^ 
February 14. 
The Velocity of Light. — Many comparisons have from time to time been 
given with a view to supply a popular notion of the rapidity with which 
light travels ; one of the latest is the following, which has been published 
by Professor Chase, of Boston. He says the velocity of light is the same as- 
would be acquired in one year by a falling body under the influence of an 
accelerating force equivalent to the force of gravitation at the earth’s surface, 
viz. 32 1-6 X 86,400 x365i-^ 5,280 = 192,254 miles per second. 
Efflorescence and Hydration find a very physical explanation in a paper 
published by M. Debray, who states that a hydrated salt has for each tem- 
perature a tension of dissociation which is measured by the elastic force of 
the aqueous vapour which it emits at this temperature. Admitting this, 
the phenomena of efflorescence and hydration are easily explained. A salt 
effloresces when the tension of its water vapour is greater than that of the^ 
aqueous vapour existing in the atmosphere. A dry salt becomes hydrated 
when the tension of the aqueous vapour contained in the atmosphere is- 
greater than that which the salt emits at the same temperature. Hydrous 
salts which do not effloresce owe, then, this property to the fact that the 
tension of the aqueous vapour emitted by them at ordinary temperatures is 
always inferior to that commonly possessed by the atmospheric aqueous 
vapour. These same salts effloresce when placed in an atmosphere where 
the elastic force of the aqueous vapour contained in the air is less than that 
which they emit. 
Sir David Brewster'' s Writings. — Some idea of the industry of the great 
philosopher who has just gone from among us may be gathered from the 
following lists, in which a record of some of his labours is briefly given. 
His essays have been contributed to the Transactions of the Boyal Societies 
of London and Edinburgh, and those of the Boyal Irish Academy, to the 
Edinburgh Encyclopeedia, the Edinburgh Philosophical Journal^ the Edinburgh 
Journal of Science^ the Philosophical Magazine (of which Sir David was one 
