APPARATUS 



99 



by a photographic plate on which the spectral lines or bands are 

 recorded. In Fig. 37 (p. 69) is shown a spectroscope with 

 one prism, and a large instrument with several prisms. The 

 latter has a camera attached to the telescope and at the other 

 end, next the slit, is an arrangement for producing electric 

 sparks or a discharge through a tube containing gas or other 

 materials which when thus ignited emit light. 



Other Instruments are required for special purposes. The 

 Refractometer is an instrument for determining the refractive 

 index of transparent substances, and one form of it is a kind of 

 spectroscope. 



The Polarimeter, of which again there are several forms, is 

 used for measuring the angle through which the plane of polarisa- 

 tion of a ray of polarised light is turned, to the right or left, 

 when the light is made to traverse a layer of known thickness of 

 a substance which has the property of circular polarisation. This 

 instrument is much used in connection with the study especially 

 of organic compounds (see Stereo- chemistry), and it is almost 

 always applied to liquids. Solids which have to be examined 

 are dissolved in an appropriate solvent. 



The Microscope is occasionally used for examination of pre- 

 cipitates or other fine powders to ascertain if they exhibit 

 crystalline structure, but it is most frequently employed in the 

 study of micro-organisms, especially in connection with fermenta- 

 tion. In the metallurgical laboratory the microscope is much 

 used in the examination of steel and other metals. 



The Colorimeter is an instrument by which a comparison can 

 be made between the colour or depth of tint of some liquid and 

 a column of equal length of a standard solution. This com- 

 parison is often applied to the case of drinking water, and 

 affords a useful indication of contamination by organic matter 

 derived from the surface drainage of land or from streams which 

 have passed through peaty or other vegetable deposits. 



The Goniometer is used for measuring the angles of crystals. 

 The instrument in its original simple form was described by 

 Wollaston in 1809. It is much to be regretted that the gonio- 

 meter is not to be met with in many chemical laboratories. This 

 is perhaps due to several circumstances. Up to comparatively 

 recent times the measurement of crystals has been applied 

 almost exclusively by the mineralogist, for the mere recognition 



