NATURAL PHILOSOPHY. 189 



results, stating that we may now for the first time feel that we 

 have a theory of the tides. 



Transmission of Gases through Colloid Substances. — In a recent 

 lecture at the Royal Institution, Dr. Odling said, in regard to the 

 transmission of gases through India-rubber, etc., that the "gas 

 appears to be condensed at the nearest surface, and to pass 

 through the pores of the material as a volatile liquid, which evap- 

 orates on the other side. He took along glass tube, witli its up- 

 per end closed with a single thickness of calico. When the tube 

 was half filled with colored liquid, and its lower end placed in a 

 dish of water, the water in the tube ran out, because of the rapid 

 passage of air through the holes in the calico. But when the calico 

 was wetted with water, the column of liquid was sustained in the 

 tube, as air could enter then only by dissolving in the water and 

 evaporating on the other side, — a very slow process. Ammonia, 

 being very soluble in water, passed through quicker ; which the 

 lecturer proved by inverting ajar of ammonia over the wet calico, 

 thus causing the liquid in the tube to descend more rapidly. 

 Liquid ammonia dropped upon the wet calico was also seen to act 

 more vigorously than the gas which had to dissolve in the water 

 before it began to pass." 



A New Pyrometer. — M. A. Lamy, in the "Coraptes Rendus," of 

 Aug. 2, 1869, describes the advantages of this instrument at 

 length. It is based upon the principle that certain compounds, 

 gaseous or volatile, are decomposed in a partial and progressive 

 manner in the same measure that the temperature is elevated, 

 and that the tension of the elements of the mixture, or tension 

 of dissociation, increases with the temperature, and remains con- 

 stant at a fixed temperature. This law has been extended to the 

 case of solid substance formed by the union of two. bodies, one 

 of which is fixed, the other volatile, like carbonate of lime. The 

 pyrometer is formed of a porcelain tube, varnished upon its two 

 faces ; closed at one end and ])ut in communication by the other 

 with a tube of glass in two branches containing mercury, or 

 connected with any other manometric system. The porcelain 

 tube contains a certain quantity of Iceland spar, or simply mar- 

 ble powder. The marble powder is heated to redness, and the 

 tube is filled with dry and pure carbonic-acid gas. When such a 

 tube cools down to the ordinary temperature, the carbonic-acid 

 gas is entirely reabsorbed by the lime, and the manometer shows 

 a vacuum. It is then a true barometer when it is not used to 

 indicate high temperatures. 



Coloration of Glass under Influence of Sunlight. — M. Bontemps, 

 who is the managing director of the celebrated glass works at 

 Choisy le Roi, states, after referring to the observations on this 

 subject made by the immortal Faraday, in 1824, and MM. Gaf- 

 field and Pelouze, in 18G3 and 1867, that his observations lead tt> 

 the following results : Within 3 months after having been exposed 

 to sunlight, the best and whitest glass made at St. Gobain is 

 turned very distinctly yellow ; extra white glass (of a peculiar 

 mode of manufacture) has become even more yellow, and grad- 

 ually assumes a color known -ds pelure d''oignon; glass containing 



