302 On the Preparation of Crayons for Draivingy 



foap, not one of them fucceeded : and it muft ncceflarily 

 be fo ; for, the excefs of alkah in the foap, operating on 

 the gelatinous matter, deftroys its agglutinating quality. As 

 the crayons for which foap has been ufed are of a browner 

 caft, it would appear that this combination abftrafts the 

 oxygen from a part of the red oxyde of iron, and gives it 

 a brown tint by making it approach the ftate of martial 

 sethiops. I have remarked, that all the paftes prepared with 

 oxydc of iron, even when pure water alone is ufed, grow 

 brown on the exterior furface as they dry : this takes place 

 in a more fenfible manner when they are expofcd to the 

 action of the fun ; which feems to arife from the light ab- 

 llratling a portion of oxygen from the oxyde of iron. At 

 fome future period I fliall enter into a farther inquiry refpeft- 

 ing the chemical properties of thefe preparations ; but at pre- 

 fent I fliall content myfelf with pointing out the procefles 

 which have condantly fucceeded with me, and in fuch a 

 way that they may be put in practice any where with fuc- 

 cefs. 



The crayons compofcd in this manner have every good 

 property that can be defired ; they do not coft one quarter 

 the common price : but it muft be obfcrvcd, that their com- 

 pofition requires great nicety in regard to the quantity of the 

 materials, becaufe the leaft variation occafions confiderable 

 difference in the quality of the paftc. Particular care muft 

 alfo be taken to guard againil the errors that may arife from 

 the wafte, which is unavoidable during the courfc of the ope- 

 ration. The beft means to prevent it will be, to fix by ex- 

 periments the quantity of water and of ingredients which 

 the pounded reddle and the folutions form before the mix- 

 tures are made. By means of the following tables, which 

 exhibit the quantity of ingredients to be ufed for the different 

 kinds of crayons, it will be eafy to know what gfoportion of 

 gum, ifinglafs, or foap, muft be employed for a determined 

 quantity of reddle, or red oxyde of iron. 



Indication 



