PHYSICO-CHEMICAL PROCESSES 153 



grade or consistency be selected. As a usual thing, nowever, 

 if grease is used indiscriminately on a large scale, especially 

 on textile machinery, a noticeable increase in the friction 

 load results. 



Greases may be divided into two classes, the lime and 

 potash soaps, or high melting-point greases; and the tallow 

 base, or low melting-point greases. The first are made by 

 changing a small amount of fatty oil into a soap by means 

 of lime water, caustic potash, or other alkali, and mixing it 

 with a large amount of petroleum oil, such as engine oil. 

 Such greases have a melting point of 140 to 180 F. The 

 tallow base greases are composed of a large percentage of tal- 

 low combined with an alkali, and are brought to the desired 

 density by means of vaseline, petroleum, or petroleum oils. 

 Such greases, owing to their large content of tallow, have a 

 low melting point, usually about 116 to 120 F. 



The high melting-point greases usually require forcing 

 down between the journal surfaces by means of compression 

 grease cups. The low melting-point greases can often be 

 packed in the journal box or directly on the bearings, as a 

 low frictional heat causes them to melt, change to an oil, 

 and lubricate the bearings. 



170. Solid Lubricants. The solid lubricants, such as 

 graphite, soapstone, etc., usually have but a limited field of 

 use. A certain form of graphite lately introduced, however, 

 has been shown in experimental laboratory tests, to have 

 great lubricating value with a low coefficient of friction. 

 The great value of this new form of graphite is due to the 

 fact that crystals of graphite appear as minute scales or 

 plates, which present a very good sliding surface and thus 

 serve as a lubricant, 



