Experiments for investigating , &c, 1 1 5 



jetfed like the common clyster with sufficient force; ot^'ef- 

 wise oiled leathers should he nicely put round the joints of 

 the machine, and a wet cloth round the injecting pipe, to 

 prevent the return of the smoke by the sides of it. Clysters 

 of carbonated hydrogen gas, or of other factitious airs, 

 might be triced. ' '' 



'' Harrowgate water takert into the stonlach, so as to indiifc^ 

 six or seven stools every morning, for four or six weeks, \ji 

 perhaps the most efficacious method in common use. A 

 factitious Harrowgate water may be m^lde probably of greater 

 efficacy than the natural, by dissolving one ounce of marine 

 «alt, (called bay salt,) and half an ounce of magnesia Glau- 

 ber's salt, (called Epsom salt, or bitter purging salt,) in 

 twenty-eight ounces of water. A quarter or half a pint of 

 this is to be taken every hour or two hours in the morning, 

 till it operates, with a tea-spoonful of a solution of liver of 

 sulphur, which is to be made by putting an ounce of hepar 

 sulphuris into ha! fa pint of water." ' 



[To be continued.] 



XXV. ExperimeJits for investigalmcf the Cause of the co- 

 loured concentric Rings y discovered by Sir Isaac Newton, 

 hetiveen two Object-glasses laid upon one another. By 

 William Herschel, LLD. F.R.S, 



■ ' ' [Continued from p. 90.] 



XV. Of the sudden Change of the Size and Colour of the 

 Mings in different Sets. 



Wme^ two sets of rings are viewed vrhich are' dependent 

 upon each other, .th<^ colour of their centres and of all the 

 rings in each set, may be made to undergo a sudden change 

 by the approach of the shadow of the point of a penknife or 

 other opake slender body. To view this phsenomenon pro- 

 perly, let a 16-inch double convex lens be laid upon a piece 

 of looking-glass, and when the contact between them has 

 been made to give the primary set with a black centre, thiat 

 of the secondary will be white." To keep the lens in this 



H 2 contact. 



