44 LAPS 



The slices are still further reduced in thickness by grinding with fine emery and 

 water on a load ' lap,' which is made to revolve on the spindle D. The lap is 8 inches 

 diameter, and about |ths of an inch thick in the centre, cast with rounded edges and 



1329 



1330 



slightly convex sides ; this form facilitates the grinding of a uniform thinness, there 

 being always a tendency on a flat surface (which soon wears hollow) for the edges of 

 the section to grind away before it is sufficiently thin. One side of the section can 

 easily be ground and finished by holding in the hand ; and this being done, it must bo 

 cemented with hard Canada balsam to a small square of plate glass, in order to grind 

 the other side, which operation must be carefully carried on until the structure appears 

 distinct and well defined. The finish requisite is best given by careful rubbing on the 

 flat surface of a hone-stone until all traces of the lines of grinding and scratches are 

 removed. . 



Thin sections for microscopic study are then usually re-mounted in Canada balsam 

 under a glass cover in the ordinary way. 



LAPIS LAZULI. A silicate of soda, lime, and alumina, with the sulphide of 

 iron and sodium in minute quantities. This beautiful mineral is found in crystalline 

 limestone of a greyish colour, on the banks of the Indus, and in granite in Persia, 

 China, and Siberia. 



The finest varieties are highly esteemed, being employed in the manufacture of 

 costly vases. It was also the source from which the beautiful pigment ultramarine, 

 was obtained, but this colour is now prepared artificially at a very cheap rate. See 

 ULTBA.MABINE. 



LAPIS OLLARIS, or Pot-stone. An impure form of steatite. 



LAPS. Metal polishing-wheels. Metal wheels or laps made of nearly every metal 

 and alloy in common use, have been more or less employed in the mechanical arts as 

 vehicles for the application of several of the polishing-powders. But of all laps, not- 

 withstanding their variety, those of lead, slightly alloyed, and supplied with powdered 

 emery, rendered the most conspicuous service. Generally the plane, or flat surface of 

 the lap, is employed ; at other times the cylindrical edge, as by cutlers ; but the portion 

 actually used in either case is called the^acfi of the lap. There are several kinds of laps. 

 The lap is in some cases a thin disc of metal, fixed by means of a screwed nut against 

 a shoulder on the spindle, but it is better with lead laps to employ an iron plate cast 

 full of holes to support the softer metal. The casting mould may in this case be either 

 an iron disc, with a central screw to fix the iron centre plate at the time of pouring, or 

 the mould may be made of sand and in halves, after the usual manner of the foundry. 



