_Black Japan._--Boil together a gallon of boiled linseed oil, 8 ounces of amber, and 3 ounces of asphaltum. When sufficiently cool, thin it with oil of turpentine. _Brunswick Black._--Melt 4 lbs. of asphaltum, add 2 lbs. of hot boiled linseed oil, and when sufficiently cool, add a gallon of oil of turpentine. The following is from _Humphrey's Journal_, Vol. viii, number 16. _Black Varnish._--I generally purchase this from the dealer; but I have made an article which answered the purpose well, by dissolving pulverized asphaltum in spirits of turpentine. Any of the black varnishes can be improved by the addition of a little bees'-wax to it. It is less liable to crack and gives an improved gloss. Before closing this chapter, it has been thought advisable to remark, that one of the most important departments of Photography is the practice of its chemistry. Many of the annoying failures experienced by those who are just engaging in the practice of the art, arise from the want of good and pure chemical agents, and the most certain way to avoid this, is to purchase them only from persons who thoroughly understand both their nature and mode of application. As many who may read this work might wish to know the prices of the various articles employed in the practice of the processes given, they can be informed by addressing the author, who will furnish them with a printed Price List. PRACTICAL DETAILS OF THE POSITIVE OR =AMBROTYPE PROCESS.= CHAPTER IV. LEWIS'S PATENT VICES FOR HOLDING THE GLASS--CLEANING AND DRYING THE GLASS--COATING--EXPOSURE IN THE CAMERA--DEVELOPING--FIXING OR BRIGHTENING--BACKING UP, &C. Manipulations. [Sidenote: MANIPULATIONS.] Under the head of manipulations I give the method I employ, and avoid confusion by omitting all comments upon the thousand suggestions of others. The glass is to have its sharp edges and corners removed, by drawing a file once or twice over it. The article used for holding the glass is called a vice. This vice is firmly secured to a bench. [Since the foregoing pages have been in type there has been introduced into market a new patent vice, adopted both for glass and plate blocks. I find it, although a little more expensive, an article better suited to the wants of the operator or amateur. It is called Lewis's Patent Glass Vice.]