GLASS AND CEMENT. 319 



The Cincinnati meeting of the American Association for the Advancement 

 of Science, really the American Congress of Science, was a most important one, 

 whether we view it in reference to the numbers in attendance, the high standing 

 of the members in their various specialties, or the bearing upon science and 

 real life, of the subjects discussed. Considerable space has been devoted by the 

 Review to the Proceedings of this meeting, still we have found it possible to give 

 but the barest synopsis, much important matter having necessarily been omitted. 

 After the adjournment the members took their accustomed excursion which 

 afforded the rest and recreation so much needed after severe professional duties. 



CHEMISTRY. 



GLASS AND CEMENTS. 



From the proceedings of the Polytechnic Club of the American Institute, 

 March 24th, we clip the following paper upon "Glass and Cements," by Dr. 

 John Phin, of the American Journal of Microscopy, which will be of very general 

 interest to a large proportion of our readers : 



Cements are to be divided into four classes according as they dry, congeal 

 by oxidation, harden by cooling, or "set" by other chemical changes. First 

 are those which harden by evaporation. Under this head may be classed paste, 

 mucilage and thfir varieties. Glues to a certain extent dry. 



The second class includes the oils. These are said to dry, but it is not by 

 evaporation. They lose nothing, but absorb oxygen from the air. The cement 

 weighs more after hardening than when first applied. Cements which congeal 

 by oxidization cannot be treated in the same way as those of the first class. They 

 require a larger time to handle. The hardening goes on from the outside inward. 

 For example, mend a piece of porcelain with one of these cements. Test it in a 

 few days, and although the outside will be hard, the inside will not appear to 

 have dried in the least, and will have no tenacity. Leave it for six months, and 

 it will be very strong. 



Thirdly, we have those cements which harden by cooling. These, instead 

 of gaining their strength slowly, like those of class two, become hard at once. 

 Shellac is a good example of a cement of this kind. China put together with 

 melted shellac is extremely strong. 



A fourth class of cemerts may be represented by plaster of Paris. This is 

 the type of an extensive class, including the whole line of mortars and hydraulic 

 cements, on which depend our great engineering works, and even the houses in 

 which we live. It forms a chemical compound combination with water first, 

 and then more slowly hardens by drying, a part of the water evaporating. 



