School Science an& flftatbematfcs 199

The apparatus is continuous in action and completely exhaustsall of the acid used. It has been thoroughly tested, with highlysatisfactory results, for the generation of hydrogen sulphide,hydrogen and carbon dioxide. In generating carbon dioxide theaction of the acid on the marble is so rapid that the tube, C, maybe left open at the upper end without danger of a great excess ofacid entering the lower compartment. The action of the acid onpure zinc or on ferrous sulphide being considerably slower, it wasfound better to somewhat limit the flow of acid by means of theclosed tube with a small hole, in the manner described. When purezinc in sticks is used it is advisable to add a little copper sulphateto the first charge of acid.A very simple device (not new) which can be highly recom-

mended consists in placing between the generator and wash bottlea constricted tube or capillary which limits the flow of gas to acertain maximum. This not only prevents a waste of gas, butalso gives a more uniform stream. The new gas generator maybe obtained from Bausch and Lomb, Chicago.

NOTE ON ETCHING WITH HYDROFLUORIC ACID.BY NICHOLAS KNIGHT,

Cornel! College.In the interesting and practical experiment o’f etching on glass

with hydrofluoric acid, it is desirable to have the entire surfacecoated with a uniformly thin layer of wax. To obtain this result,the writer for a number of years has kept an ordinary coppersteam bath, six inches in diameter, nearly filled with paraffine.This is carefully heated to avoid burning until it is all melted. Theglass object to be etched is immersed in this liquid and quicklywithdrawn, when both sides are uniformly covered with a thinlayer. The etching is so satisfactorily accomplished that manytimes the students are desiro’us of repeating the experiment, andthey etch beautiful designs on various articles, watch crystals,paper weights and similar objects. Thus they have a perpetualreminder of the days spent in the fascinating realms of experi-mental chemistry.