GLASS AND GLAZING. 



533 



means of escape are greatly lessened. 

 Early morning ventilation is of great 

 importance, whatever mode of glazing 

 be adopted ; and it is probably the 

 lesson taught us by the scorching and 

 burning complained of that has directed 

 that attention to ventilation, not only 

 during the day, but during the night 

 also, which has led of late years to a more 

 perfect study of that important subject. 

 In all such houses as are built upon the 

 best principles, and glazed with large- 

 sized panes, let the glass be what it may, a 

 constant system of ventilation must be 

 maintained during both day and night ; 

 and where this is duly attended to, we 

 doubt not all the evils of burning and 

 scorching will disappear. 



Notwithstanding all this, it behoves us 

 to be upon our guard against an inferior 

 quality of glass ; and this object is first 

 secured by dealing with respectable (not 

 cheap) firms in our purchases : and, second- 

 ly, to exercise our own judgment by ap- 

 plying some of the many tests by which 

 the quality of glass may be determined, 

 and of which the following may be given 

 as one both simple and certain : Take a 

 square of glass and hold it up to the sun, 

 so that the light passing through it may 

 fall on a white surface — a sheet of paper, 

 for example — carrying the glass from a 

 distance of a few inches to 8 or 10 feet. 

 If any bright or luminous spots or stripes 

 are shown on the white surface, it is un- 

 safe to use, and should be rejected. This 

 test should be made by presenting both 

 sides of the glass to the sun alternately. 

 Although to inadequate ventilation may 

 be chiefly traced the evils complained of, 

 it may not be uninteresting to glance at 

 some of the other causes to which these 

 evils have been attributed ; and, first, 

 because we think there is a good deal in 

 the argument, we may state that Dr Lind- 

 ley was of opinion that the angle the roof 

 may be placed at has a good deal to do 

 with the evils discovered in the use of 

 sheet glass, as at first manufactured, as 

 well also as of its variability in quality. 

 Regarding the first of these he says, in 

 "The Gardeners' Chronicle :"— " If, for 

 instance, the roof forms such an angle 

 that the sun can strike it perpendicularly 

 near the middle of the day, at the season 

 when the leaves are young and tender, an 

 effect may be produced by the rays of 



light, however imperfectly concentrated 

 by the irregular lenses, which effect would 

 not be produced by the same glass placed 

 at another angle of elevation, as plants 

 may be entirely out of the foci of the 

 lenses. Hence it will follow that in one 

 case its use will be condemned, while in 

 another it may be found satisfactory." 



The same authority last quoted thus 

 explains part of the process of manufac- 

 ture : "From the way in which sheet 

 glass is made, it must necessarily have nu- 

 merous concavities and convexities, some 

 of which, or many, as the case may be, 

 have the power of concentrating the rays 

 of light enough to burn the leaves of 

 plants. The process of splitting and flat- 

 tening cylinders, whose exterior circum- 

 ference is \ of an inch greater than the 

 inner, must cause such irregularities to 

 occur, with whatever degree of care the 

 flattening is conducted. If sheet glass 

 were simply irregular, or wavy, or uneven, 

 its thickness being uniform, this concen- 

 tration would not take place; but its 

 thickness is so variable, from the effects of 

 ' cockling,' that it may be compared to a 

 layer of meniscus lenses or spectral glasses 

 fused together j and these, from their 

 large size, and the small difference be- 

 tween the diameter of their inner and 

 outer curves, will have long foci. When 

 the cylinder is spread open into a flat 

 sheet, the two surfaces become of equal 

 width, the glass adjusting itself by the 

 expansion of the inner or smaller surface, 

 and by the contraction of the outer or 

 larger surface. In this operation is formed 

 what the manufacturers call ' cockles,' 

 producing that uneven puckering appear- 

 ance which is the peculiar characteristic 

 of sheet glass. Of these cockles some are 

 circular, and form lenses of considerable 

 power. It is said, indeed, that the effect 

 of burning is observed far beyond the 

 focus of any possible lens in the glass; 

 but this is a mistake, as will be readily 

 seen by watching the luminous spots 

 formed on leaves beneath sheet glass in a 

 bright sun." 



The process of flattening, as carried on 

 a number of years ago, was thus per- 

 formed : The cylindrical glass or "muff" 

 being cut longitudinally down the middle, 

 was then placed in a heated oven, the bot- 

 tom of which was as nearly level and 

 smooth as possible, as upon this, we 



