GREENHOUSE 



GREENHOUSE 



1403 



other colors, dark blue glass letting through but 18 

 per cent. In connection with the matter of tint, it 

 should be noted that some glass, especially clear 

 white glass purified with arsenic acid, or that in which a 

 large amount of potash is used in proportion to the 

 amount of lime employed in manufacture, becomes dull 

 after long exposure to the weather, the dullness being 

 occasioned by the efflorescence of salts contained in 

 the glass. Before this disintegration has proceeded too 

 far, the crust or efflorescence may be removed with 

 muriatic acid. 



The strength of glass depends upon its thickness and 

 upon the thoroughness of the annealing. Glass is 

 annealed by passing through a series of ovens, where it 

 is raised to a high heat and then gradually cooled; 

 whatever toughness and elasticity the finished product 

 may contain is due to this process. The thickness of 

 glass varies, not only with grades (single and double 

 thick), but also more or less within the grades, and 

 even in different parts of the same pane. Single thick 

 glass is too thin for use in greenhouses; in selecting any 

 glass for such a purpose it should be examined pane by 

 pane, and all showing marked variation in thickness, 

 either between panes or in different parts of the pane, 

 rejected. A pane of varying thickness is much more 

 liable to breakage from climatic changes or sudden 

 shocks than one which is uniform in this regard. From 

 the foregoing statements it will be seen that, in general, 

 the ordinary double-thick green glass is best as regards 

 both tint and strength, green glass being less liable to 

 change in tint than white, and the double-thick being 

 the stronger grade. By green glass is meant simply the 

 ordinary sheet glass, the green color of which is notice- 

 able only at the cut edge. 



It has long been a common opinion that such visible 

 defects in sheet glass as the so-called "bubbles," 

 "blisters," and "stones," produce a focusing of the 

 solar rays passing through them, thus burning the 

 foliage of plants grown under glass containing these 

 defects (Fig. 1759). This view has been held by glass 

 manufacturers and horticulturists alike, and seems not 

 to have been contradicted publicly until 1895 (Bulletin 

 No. 95, Cornell University Agricultural Experiment 

 Station, page 278). In view of the erroneousness of 

 this theory, it is rather remarkable that it should have 

 gained such prevalence. Nearly all bubbles and blis- 

 ters are thinner in the middle than at the periphery, 

 being thus concave rather than convex lenses, and 

 actually diffusing the rays of light passing through 

 them rather than producing destructive foci. While it 

 is true that sand-stones or knots in glass may produce 

 foci, these points of focus scarcely ever exist more than 

 a few inches from the surface of the glass; consequently, 

 these defects can do no damage when occurring in 

 roofs several feet distant from the growing plants 

 below. 



The only full and complete series of experiments 

 on this subject in this country (conducted at the 

 Cornell University Agricultural Experiment Station, 

 the Physical Laboratory of Cornell University, and a 

 glass factory in Ithaca, New York, but yet unpub- 

 lished) shows the true cause of the burning by glass to 

 be the variation in thickness of the entire pane, or a 

 portion of it, thus producing a prismatic or lens-like 

 effect (Fig. 1760), which causes a more or less distinct 

 focusing of the sun's rays at distances varying from 5 

 or 6 feet to 30 feet, or even more, from the glass. 



This defect usually occurs along the side or end of 

 the pane and is not visible to the eye, but it may be 

 detected easily by using the micrometer caliper or by 

 testing in the sunlight. It may be found in all kinds of 

 glass, and is caused by the glass-blower while reducing 

 the upper or pipe end of the cylinder from which sheet 

 glass is made, thus facilitating the removal of the 

 "cap" or neck end of the cylinder, by which it is attached 

 to the pipe while being blown. The defect, as already 



stated, is one which may be found in all grades and 

 qualities of sheet glass, of both foreign and domestic 

 manufacture. The fact is well known that differences 

 in the thickness of spectacle lenses, which are imper- 

 ceptible to the eye, may produce sufficient refraction 

 to vary materially the direction of rays of light passing 

 through such lenses, and it is not difficult to see that 

 the same effect may be produced by similarly imper- 

 ceptible variations in the thickness of sheet glass. That 

 this is the case has been conclusively shown by the 

 series of experiments mentioned above. These also 



POINTS OF 

 REFRACTION 



CONVERGENT RAYS 



FOCAL DISTANCE '0 FT. 



1760. Refraction of light rays by an irregular pane of glass. 



show that burns on plants caused by defective glass 

 roofs occur in lines and not in isolated spots, burns of 

 the latter description being usually the result of a 

 weakening or deterioration of tissue, due to careless- 

 ness in the matter of ventilation, humidity of the 

 atmosphere, water, and temperature of greenhouses, 

 rather than to defects in the glass. 



If, therefore, it is not possible to secure glass of 

 uniform thickness with certainty, it may be found 

 cheaper and often fully as satisfactory to purchase the 

 lower or common grades of double-thick glass, using 

 in the roof only those panes which show, after testing 

 in the sunlight for foci, an entire lack of the prismatic 

 character which makes them dangerous to plants 

 grown under them. j. c. BLAIR. 



Greenhouse heating. 



In all sections in which the temperature drops below 

 the freezing point, it is necessary to provide some 

 artificial means for heating greenhouses. Nearly all 

 modern structures are warmed either by steam or hot 

 water, although hot-air flues are occasionally used. 

 While hot water is preferred for small ranges of glass, 

 as it can be depended upon to furnish an even degree 

 of heat when left for a number of hours, steam is very 

 generally used for extensive plants, as the cost of piping 

 the houses is much less than when hot water is used. 

 Steam boilers require more attention than hot-water 

 heaters, but when there is more than 10,000 or 12,000 

 square feet of glass, it is best to have a night fireman 

 and watchman, and the extra expense can be made up 

 by the saving in the cost of fuel, as it will be possible 

 to use a lower grade of coal. Under these conditions the 

 cost of running a steam plant will be as low as with hot 

 water, but in small houses, where hard coal is used, 

 and the fires receive no attention for six to eight hours 

 during the night, hot-water heaters will be cheapest 

 to operate, and will be most satisfactory. Some of the 

 up-to-date ranges of the largest size make use of hot 

 water and are able to secure a perfect circulation by the 

 use of steam or electric pumps, which also make it pos- 

 sible to reduce the size of the piping, and as a higher 

 temperature is maintained in the water, the amount 

 of radiation required and consequently the cost of 

 piping the houses is reduced practically to that in steam 

 systems. Similar results can be secured in closed sys- 



