Roof |'r(o'~ffsh.
[Jour. Frank. Inst., hie in alcohol, and in water to a large extent. The fbllowing experliimnts arc worth a trial. Dissolve one gr~:in in two drachms of water~ :tnd expose a collo(lio-bromide plate fbr half the time necessary with the ordinary alkaline developer, and apply this quantity of liquid with one drop of strong ammonia. The linage will appear quietly, and d.~w all detail, but on fixing will he rather too tbeble; it will take silver intensification, however, perfixMy. Double the quantity of hydro-kinonc, and again develop a plate, and it will be found that the intensity it of a medium character, whilst hv trebling it, it will be found that proper density is secured. With gelatine plates the same experiment may be repeated, a less quantity being apparently requisite to give proper printing density, th'm with collodion films. On fixing the plates they will be found perfectly tYee from any w.'il caused by a reduction where no reduction should be, and they will give 1)rilliant l)rints.
[t nmy t)e asked it' the hvdro-kinone has any adwmtage over the .r(linary alkaline developer. I am inclined to think it has--in that the faintest trace of the action of light is made apparent, and is not ,lestroycd 1)v the soluble bromide. This simuld I)e a decided gain, :rod on testing it against fi~rrous oxalate (without 1)romide) in gelatine plates, it seems as if this were borne out. With (x~llodion plates it has a decided advantage, and it tnav bc worth a trial. A curious experiment is to lwepare a collodion plate, wash it, and pour over it a solution of hydro-kinone and ammonia, and then expose in the camera; it will be fimnd that developnmnt commences during the exposure ; whereas such an attempt nmde with ordinary alkaline or ferrous .xalatc developer would prove a fhilure. The great objeetion to the use of this compound it its price; it is expensive, but doubtless any demand for it wouhl quickly re(luee it to nearly the same price as pyrogallie acid. It is at present more :t chemical euriosity than of :lny commercial use, but if it should llrOVC to he a good developer, it would speedily find its way into tile price list of manufiteturlng chemists at a more moderate rate than that at which it is now shown.--Pllotwraplcic New.~.
Roof Varnish.--A varnish fiw roof:* has been recently patented in (;CVlnany which it composed of "~5 parts of clay slate, 30 parts mica sl'ttc and 35 parts rosin, all finely 1)owdered and heated with 50 parts of t:u'. -]hty. l)~d. .~.l Gca,e,'bcbhl/!. C.