68 Sciniiijic iSiotices — Chemistrrj. [Jan. 



The following communication was also read: — 

 On a new Photometer; by \. Ritchie, AM.: communicated 

 by the President. 



The principles on which the indications of this instrument 

 depend, are, that radiant heat does not pass through thick plates 

 of glass, but is conducted through them in the same manner as 

 through opaque bodies ; that light expands in the same manner 

 as heat the substances which absorb it ; and that the intensity 

 of light varies inversely as the square of the distance. Mr. Rit- 

 chie's photometer, however, differs essentially from that of Prof. 

 Leslie. Its delicacy is such that it is very sensibly aifected by 

 the light of a candle at the distance of 20 or 30 feet, while no 

 effect is produced on it by a hot ball of iron radiating a nmch 

 greater quantity of heat. When exposed to several lights at 

 different distances, it expresses their intensity according to the 

 law just stated. 



Article XIII. 



SCIENTIFIC NOTICES. 



Chemistry. 

 1 . Sdline Efflorescence upon the Surface of Bricki. 



(To the Editors of the Aunnh of Pliiloxoplnj.) 

 GEi\TLE]\JEN, Llsson Grove. 



Walking some time since with a friend in the neighbour- 

 hood of St. John's Wood Road, where a considerable number 

 of cottages in the Italian style, surrounded by gardens and 

 inclosed with brick walls, have lately been erected, our attention 

 was attracted by observing the irregular distribution of a white 

 coloured substance upon the surfaces of some of these garden 

 walls. Upon closer inspection, a pretty thick and extensive 

 efflorescence became apparent, a portion of which when scraped 

 off and applied to the tongue communicated a strong and disa- 

 greeable saline taste, A quantity of this saline substance was 

 collected, and on being submitted to a slight chemical examina- 

 tion, it soon became evident that it was almost entirely com- 

 posed of sulphate of soda, blended with minute portions of 

 muriate of lime and mao nesia. 



T 1 



J am aware that sulphate of soda has occasionally been met 

 with as an efflorescence upon old walls on the Continent and 

 elsewhere, but I do not think that it is a very common occur- 

 rence in this country ; and may it not be asked, whether the 

 solidity of walls built of such materials is not likely to be mate- 

 rially diminished .' 



The salt evidently, in the first intance, crystallizes upon the 



