474 Progress of Invention. 



acting on iodide of amyl with lepidine. Schonbein's test fluid is 

 applicable also to the detection of bases, and is of extraordinary 

 delicacy when used to that purpose, enabling us to ascertain the 

 presence of the exceedingly small amount of oxide of lead, which is 

 dissolved by water, and which is not to be rendered perceptible by 

 sulphuretted hydrogen. That used for acids is adapted to alkalies 

 by merely reddening it with an acid. 



Propulsion of Boats on Canals. — Great as has been the exten- 

 sion of the railway system, and numerous as are the advantages it 

 possesses over every other mode of conveyance, canals, at least 

 those already constructed, would retain a large portion of their 

 utility, and of their superiority, in point of economy, could steam 

 he employed generally upon them. But such has not been found 

 the case, notwithstanding the many efforts which have been made 

 for the purpose. The great disturbance caused in the water, not 

 only by every form and application of paddle-wheel, but also by 

 every adaptation of the screw propeller, causes a ruinous effect on 

 the banks. A trial is, however, being made in France of a mode of 

 using the paddle-wheel, which is found to be free from objections 

 of this kind, and which will, if not accompanied by others peculiar 

 to itself, most probably give a new impulse to canal navigation. 

 Only one paddle-wheel is used, and it works in the centre of the 

 boat, in a space which is enclosed on all sides, except at the bottom. 

 The lowermost paddles project below the keel, and are so effective 

 as to produce, it is said, a velocity equal to that obtained with any 

 other arrangement that has been tried. 



Simple Mode op rendering Resins Soluble. — Certain resins, 

 such as Calcutta copal, are not, in ordinary circumstances, soluble 

 in spirits of turpentine, benzine, petroleum, and other hydrocarbons ; 

 but they are rendered so by the application of heat. This, however, 

 is objectionable, among other reasons, from the loss of one-fourth 

 of the weight which occurs. It has been found by M. Violette that 

 this loss may be avoided by heating the resins in close and strong 

 vessels to a temperature between 350° and 400° Cent., and after this 

 cooling them. They are then soluble, even at ordinary temperatures, 

 in the above-mentioned fluids, forming excellent varnishes, which 

 may be used for even the most delicate purposes. They undergo 

 no loss of weight, and the effect produced on them is due to the 

 conjoined action of both heat and pressure. There is one circum- 

 stance, however, which, at first, may be a source of difficulty, when 

 the manufacture is carried on upon the large scale. The vessel in 

 which the resin is heated must be able to support a pressure of 

 about twenty atmospheres. The resin may be heated, and the 

 varnish made by a single process. For this purpose it is only 

 necessary to heat to the required temperature a mixture consisting 

 of one part by weight boiled linseed oil, four parts spirits of tur- 

 pentine, and one part Calcutta copal. 



Precipitation op Metals prom their Solutions va Maghestuh. 

 — Magnesium, for a long period after its discovery, remaiix d a mere 

 chemical curiosity. It was then applied to the production of ;i light. 

 suitable not only for ordinary illumination, but, which is of far 





