themselves much of this indication. In 

 general, a neat curved line fracture, and 

 even grey texture, denote good steel ; 

 and the appearance of threads, cracks, or 

 brilliant specks, denotes the contrary. 

 But the management of the forging and 

 other circumstances of manufacturing will 

 modify these indications ; and the steel 

 that is good for some purposes may be 

 less suited to others. 



The fluid into which ignited steel is 

 plunged is of great consequence. All the 

 facts seem reducible to these general con- 

 clusions. The hardness will be greater, 

 1. The hotter the steel is made, provided 

 it be not decomposed : 2. The more con- 

 siderably its temperature is lowered in 

 the cooling: 3. The shorter the time of 

 cooling : and 4. The more favourable the 

 fire or the cooling material may be to the 

 steel-making process. But the most use- 

 ful combination of hardness and tenacity 

 will be at a medium temperature in each 

 kind of steel. 



With regard to the first particular, lit- 

 tle need be said, but that the decompo- 

 sition of steel in heating will be prevent- 

 ed, and its surface somewhat improved, 

 if it be bedded in charcoal, or the cement- 

 ing compound, during the application of 

 the heat. The second and third, namely, 

 the quantity and suddenness of cooling, 

 require an attention to the doctrine of CA- 

 XOHIC, as explained under that article. 

 The cooling will be more sudden and ef- 

 fectual, the greater the quantity of heat 

 absorbed in the same time. There are 

 three circumstances which favour this 

 effect, namely, a very low temperature of 

 the body to which the hot steel is applied; 

 that it should be a good conductor of 

 heat; or that it shall assume either the 

 fluid or elastic state, which always de- 

 mand a supply of heat for their mainte- 

 nance. Thus it is found, that steel is more 

 effectually hardened in cold than in warm 

 water, and, at like temperatures, more ef- 

 fectually in mercury than in water. It 

 may also be remarked, that these two 

 fluids cool the steel by different ener- 

 gies. The water is partly converted into 

 vapour, which carries off the heat, and 

 leaves the fluid much less altered in tem- 

 perature than mercury, which acts by its 

 conducting property. This last fluid, not 

 having evaporated in the process, is found 

 to have acquired a much more elevated 

 temperature by the immersion. Oil is 

 found to harden the surface of steel much 

 more than its internal part, so that it re- 

 sists the file, but is much less easily bro- 

 ken by the hammer. This effect arises 



from its imperfect conducting quality, and 

 the elevated temperature it demands to 

 be converted into the vaporous state ; to 

 which we may also add, that a stratum of 

 coal is formed round the steel from the 

 burned oil, which still more effectually 

 prevents the transmission of the heat A 

 remarkable instance of this nature pre- 

 sented itself to our observation,in harden- 

 ing a small piece of steel two inches long, 

 and a quarter of an inch diameter. At the 

 time of ignition, the water nearest at hand 

 had been used with soap. The steel made 

 very little noise when plunged into the 

 water, and remained hot for a consider- 

 able time ; but when taken out, was found 

 to be scarcely at all hardened. It was 

 covered with coally matter ; which being 

 cleared off, and the process repeated witli 

 clean water, it became perfectly hard. 

 The heat, in both cases, was a low red 

 heat, proper for cast steel which is not 

 intended to be afterwards annealed. We 

 had very little doubt, but that alcohol 

 and the essential oil of turpentine, which 

 are good conductors of heat, and very 

 volatile, would render steel very hard, if 

 their inflammability, and the little neces- 

 sity there is for using them, were not an 

 impediment to their application. Various 

 artists avail themselves of different sub- 

 stances for the immersion of ignited steel. 

 Some use urine, others water charged 

 with common salt, nitre, or sal ammoniac. 

 Tallow and oil are used for such works 

 as are not required to be brittle, though 

 very hard, the reason of which has 

 just been mentioned; but tallow differs 

 from oil in the heat which becomes latent 

 for its fusion ; and, accordingly, solid tal- 

 low is an excellent material tor hartlening 

 drills and other small articles. It has 

 been found by Reaumur, that saline li- 

 quids produce rather more hardness than 

 common water; and, in particular, that 

 aquafortis possesses this property in tin 

 eminent degree, probably from its con- 

 ducting power; the makers of files' co- 

 ver them with the grounds of beer and 

 common salt, which assist their harden- 

 ing, and keep the surface from scorifying. 

 The mucilage of the beer supplies a coal- 

 ly matter; and the fused salt seems not 

 only to form a varnish in the fire and de- 

 fend the steel, but may also produce coal, 

 by its sudden solution in the water at the 

 time of immersion. Very small articles 

 heated in a candle are found to be hard- 

 ened perfectly by suddenly whirling them 

 in the cold air ; and thin bars or plates of 

 steel, such as the magnetic needle of a 

 compass, acquire a good degree of hard- 



