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THE POPULAR SCIENCE MONTHLY. 



and hence the more abundant the dust the 

 more dense the cloud. When the vapor in 

 the second receiver was brought by circula- 

 tion against the sides of the receiver, it 

 gradually condensed on these surfaces. The 

 density of the fog formed in common air 

 shows what a large amount of dust is pres- 

 ent every day in the air around us. But the 

 particles of fog do not represent all the dust- 

 particles in the air. If enough steam is 

 blown into a receiver full of common air to 

 produce a dense fog, and after the fog has 

 settled more steam is blown in, another fog 

 will form on the dust which still floats in 

 the air. If this is repeated a number of 

 times, a less dense and coarser-grained fog 

 forms each time, till at last no fog is seen, 

 but the condensed vapor falls as rain. These 

 dust-particles are not the motes that we see 

 in the path of a sunbeam ; for, when com- 

 mon air is passed through a flame, these 

 motes disappear, but the air still remains a 

 good medium for fogs. It is a finer kind of 

 dust which furnishes the fog and cloud nu- 

 clei. The products of combustion are fog- 

 producers, and especially the vapor from the 

 burning of sulphur. 



Gem Minerals of Canada. Although, ac- 

 cording to Mr. George F. Kunz's paper on 

 " Precious Stones, Gems, and Decorative 

 Stones in Canada and British North Ameri- 

 ca," Canada can hardly be called a gem-pro- 

 ducing country, it furnishes a number of 

 stones that are of more than passing interest 

 to the mineralogist, and of some value in jew- 

 elry and the arts. A number of gem minerals, 

 not of gem quality, are found in examples 

 of such size and perfection that they have 

 been given prominent places in cabinets, and 

 are even more prized as specimens than cut 

 stones from other localities. Their mineral- 

 ogical value gives them no small commercial 

 importance. Of such are magnificent zircon 

 crystals, occurring as individuals up to fif- 

 teen pounds in weight, and many finer ones 

 weighing a pound, as well as beautiful twin 

 crystals of the same mineral ; black titanite 

 in simple and trimmed crystals up to seventy 

 pounds each ; " vast quantities of amethyst " 

 from Lake Superior ; ouvorsovite or green 

 chrome garnet from Orford, and white gar- 

 net crystals from near Wakefield ; and apa- 

 tite crystals, one weighing over five hundred 



pounds, of great beauty, of which the rich 

 green variety, especially, would do to work 

 into ornaments similar to those made from 

 fluorite. Only a small part of the territory 

 of the Dominion has been examined with 

 reference to these stones ; and with the dis- 

 covery of new localities important additions 

 to the list may be anticipated. 



The Sliding Railway. The Chemin de 



Fer GUssanl, or sliding railway, at the Paris 

 Exhibition, according to a description by Sir 

 Douglas Galton in the British Association, 

 is based on the two principles of causing the 

 carriage to slide on a thin film of water in- 

 troduced between the sledge-plates on which 

 it rests ; and the propulsion of the sliding 

 train by horizontal columns of water acting 

 through hydrants placed at intervals on the 

 line. The system was originally designed 

 by Girard in 1861, who made a line at his 

 own private house, where he had an inclina- 

 tion of one foot in twenty. The results he 

 obtained seemed to justify the application of 

 the system in special cases on a paying basis. 

 He acquired a concession in 1869 for a rail- 

 way from Calais to Marseilles, to which a 

 subvention was afterward attached. But 

 the War of 1870 resulted in the destruction 

 of the railway by the German army, and in 

 the death of M. Girard in 1871. In 1885 

 M. Barre purchased the drawings left by 

 M. Girard, and introduced an improvement 

 which he considered would make the system 

 more workable. A line on this improved 

 system was established in the Paris Exhibi- 

 tion, about two hundred yards long, and 

 trains were run upon it. 



Properties of the Kola-Nut. Kola-nuts, 

 or the seeds of Sterculia acuminata, are allied 

 in composition to cocoa, coffee, and tea, but 

 contain a relatively large amount of caffeine. 

 They are credited with strong tonic and 

 nervous stimulant properties ; with counter- 

 acting and removing the sense of exhaustion 

 after fasting and fatigue ; with having an- 

 tagonistic reaction to alcohol ; and with a 

 purifying influence on water. Their value 

 as a therapeutic and dietetic agent has been 

 tested by Surgeon R. H. Firth, who concludes 

 that kola is not a food ; that it increases total 

 urinary water, has a stimulant action on the 

 nervous system, temporarily strengthens the 



