320 Tlie Philosophical Society of Glasgow. 



to deal with sea-water, which contains 100 grains per gallon of sulphate 

 of Hme, the case is very different ; and it becomes a question of calcula- 

 tion whether the blowing-off method or the chemical method is the 

 more economical. The calculations given in Mr. J. R. Napier's paper 

 appear to me quite correct ; but I would suggest one or two considera- 

 tions in addition to those taken into account by Mr. Napier. The 

 crust is a bad conductor of heat, and some loss of heat is likely to occur 

 from this circumstance. Again, the slow conduction of heat through 

 the crust must render the boiler-plates and tubes much hotter than 

 they would be if in immediate contact with water — thus causing them 

 to wear away faster than they would if kept cool. Then there is said 

 to be some danger of explosion, arising from the peeling off of the crust, 

 and the consequent exposure of the highly heated metal below. On the 

 other hand, it will be found in practice that the exact amount of alkali 

 calculated from the sulphate of lime wUl not suiBce for the prevention 

 of crust, but that a very considerable excess will be required. Again, 

 the soda wUl elevate slightly the boHing 'point of the Uquid ; but 

 probably the difference of temperature would be trifling. 



Upon the whole, it appeai-s that the saving effected, even in long 

 voyages, would not be found in practice quite so great as Mr. Napier's 

 calculation indicates. The use of efficient brine-chests, such as he 

 described, would seem to be more economical, and involve less trouble, 

 than the chemical method. WhUe, therefore, carbonate of soda is 

 invaluable as a crust-preventing agent for boUers using river-water, its 

 application to marine boilers would not probably be attended with a 

 saving of money, except in very rare instances. The question can be 

 decided only by practical trials ; and it is to be hoped that Mr. Napier 

 will continue his investigations until he is able to furnish satisfactory 

 and rehable results. 



Dr. Wallace next read a paper " On Electrical Discharges in Rare- 

 fled Media," illustrated by numerous experiments. The Ruhmkorff 

 coil employed in the illustrations is said to be the largest ever con- 

 structed, the secondary coil containing about eight miles of wire. By 

 electric discharges in rarefied air and other gases and vapours, the 

 appearance, in miniature, of the aurora boreaUs was exhibited, and stra- 

 tified bands of Ught were produced. In rarefied air, obtained by means 

 of the air pump, the current presented a reddish-purple glow of Hght, 

 divided into bands or rings. In different gases and vapours, rarefied in 

 the same manner, the glow of light assumed a variety of colours. The 

 apparatus was furnished by Mr. J. W. Stone, Philosophical Instrument- 

 maker. 



