560 PHOTOGEN 



heated in vertical or horizontal iron retorts, the tar being received through a very 

 wide worm into large tanks. Some manufacturers use vertical, and others horizontal 

 retorts ; it is also common to distil the coals by the heat produced by their own 

 combustion. If the latter process be employed, the arrangements for condensing the 

 product must be very perfect, or great loss will be sustained, owing to the air which 

 supports the combustion carrying away a considerable quantity of the hydrocarbons. 

 This power of air to saturate itself with vapours is of great importance in the 

 economy of all processes where the distillation of one portion of substance is carried 

 on by the heat evolved by the combustion of .another. It is not uncommon in prac- 

 tice, where the cylinders are horizontal, to place the coal or other matters to be 

 distilled in semicylindrical trays, which are capable of being inserted into the retorts, 

 and also of being removed to make way for another charge at the completion of the 

 operation. 



The tar obtained by any of the above processes is to be redistilled : the lighter 

 portions form (when purified by means of sulphuric acid and alkalis) the fluid known 

 in commerce as 'Boghead naphtha.' (See NAPHTHA, BOGHEAD.) In Germany and 

 some other places, it is usual to divide the distillate from the tar into two portions, 

 one being for the preparation of photogen, and the other for ' solar oil." This division 

 is made as the fluid runs from the still ; the more volatile constituting the photogen, 

 and the less the solar oil. 



The process of purification is the same in both cases, namely, alternate treatments 

 with concentrated sulphuric acid to remove the highly-coloured and odorous consti- 

 tuents of the crude distillate, and washing with an alkali to remove carbolic acid and 

 its congeners ; also that portion of sulphuric acid which remains suspended in the 

 naphtha, and the sulphurous acid produced by the decomposition of a portion of the 

 sulphuric acid by the carbon of certain easily-decomposed organic matters in the crude 

 distillate. This decomposition of the sulphuric acid happens thus : 



2S0 3 HO + C = 2S0 2 + 2HO + CO 2 

 (2H-S0 4 + C = 2SO 2 + 2H 2 O + CO 2 ). 



There is another advantage in the treatment of the fluid by alkalis, inasmuch as 

 some sulphide of hydrogen, and probably other foetid sulphur-compounds, is decom- 

 posed and the resulting products removed. 



In preparing photogen from any of the sources enumerated, much must be left to 

 the discretion of the manufacturer, both as regards the apparatus and the chemical 

 processes. In some instances the solar oil and photogen are with advantage prepared 

 separately ; but in this country it is more usual to mix the heavy and light oils 

 together, so as to produce a fluid of medium density and volatility. It must bo 

 remembered that while the more volatile hydrocarbons confer extreme inflammability 

 and fluidity, they are at the same time more odorous than the less volatile portion of 

 the distillate, which is the true paraffin oil. 



The more odorous impurities in photogen appear to be easily susceptible of oxida- 

 tion. This is evident from the facility with which foully-smelling photogen loses its 

 offensive odour in contact with bichromate or manganate of potash, or even animal 

 charcoal. Their exposure to air even greatly improves the odour, and a recently- 

 distilled photogen, which is very unpleasant, becomes comparatively sweet if kept in 

 tanks or barrels for a few days. The same thing happens with many essential oils, 

 such as those of peppermint, cloves, &c. The presence of sulphurous acid in photo- 

 gen may be instantly detected by shaking a little in a test-tubo with a few drops of a 

 very weak solution of bichromate of potash ; if sulphurous acid be present, a portion 

 of the chromic acid will be reduced to green oxide, which will instantly betray the 

 presence of the reducing agent alluded to. 



Photogen often shows the phenomenon of dichroism ; but the more it is purified 

 by acids, the more feebly is the coloration by reflected light observed, and if the 

 less volatile portion of the distillate be rejected, the property alluded to will not be 

 perceived. 



In distilling the heavy oils or tars produced by distilling Boghead coal or other 

 photogen-yielding substances, it is particularly to be observed that the worms or 

 other tubes proceeding from tho stills, if of too small diameter, are liable to be- 

 come choked up with paraffin ; this, if unobserved, might lead to serious results. It 

 is very convenient to have a steam-pipe inserted into tho worm-tubes or condensiug- 

 tanks, to enable the water to be heated to such a point as to melt any solid matters 

 in the worms, and allow them to be washed into the recipient by the fluids distilling 

 over. 



The following Table by Wagenmann will be found of great importance to those who 



