C. GENERAL PHYSICS. 139 



by weight, of hyposulphite of soda in fifteen parts of boiling 

 water, and gently pour the same into a tall glass jar so as to 

 half fill it, keeping the solution warm by placing the glass 

 in hot water. Then dissolve one hundred parts, by weight, 

 of the acetate of soda in fifteen parts of hot water, and care- 

 fully pour it into the same glass, inclining the latter so that 

 the second solution may gently flow upon the first, and form 

 an overlying layer without mingling with it. When cool 

 there will be two supersaturated solutions. If now a crystal 

 of hyposulphite of soda be attached to a thread and care- 

 fully passed into the glass, it will traverse the acetate solu- 

 tion without disturbing it, but on reaching the hyposulphite 

 solution will cause the latter to crystallize instantaneously 

 in large rhomboidal prisms with oblique terminal faces. 

 When the lower solution is completely crystallized, a crystal 

 of the acetate of soda similarly lowered into the upper solu- 

 tion will cause it to crystallize in oblique rhombic prisms. 

 The appearance of the two different kinds of crystals, and the 

 method of their formation by the selection of the disturb- 

 ing crystal, forms an interesting philosophical experiment. 

 Though not mentioned in our source of information, it would 

 be well to observe the precaution of covering the surface of 

 the upper solution with a thin layer of oil, to protect it against 

 disturbance and the falling in of dust-particles, by which the 

 crystallization will readily be set up, and the foregoing ex- 

 periment anticipated quite unexpectedly. 



FRICTIONAL RESISTANCE OF AVATER TO MOTION OF VESSELS. 



In the report to the Lords Commissioners of the Admiral- 

 ty on the experiments for the determination of the frictional 

 resistance of water to movinc: surfaces under various condi- 

 tions, it is stated that a number of surfaces were experi- 

 mented with, the experiments being conducted by drawing 

 known areas of the given body through the fluid. The re- 

 sults may be expressed generally by a formula based npon 

 the assumption that the resistance offered to the particles 

 of fluid is purely dynamic, dependent upon the weights of 

 the particles and the velocities imparted to them. Conse- 

 quently, for lengths of surface above fifty feet, the increase 

 of the friction per square foot of every additional length is 

 so small that it will make no very great difference in our es- 



