GLASS STRUCTURES 47 



provement over sash houses, and from the crude green- 

 house of that period the present-day modern house has 

 been evolved. 



This evolution has been gradual, and inasmuch as 

 students of light factors have learned much during the last 

 decade, the present-day house represents a high degree of 

 light efficiency. Gradually, the heavy, wooden frame has 

 been replaced by lighter wooden material, and this, in 

 turn, has given way to steel frames. Wood is not an ideal 

 material for greenhouse construction. It decays too 

 rapidly under the conditions which prevail in these houses. 

 Decay in wood tissue is caused largely by wood-destroying 

 fungi which develop rapidly under warm, moist conditions 

 such as are found in greenhouses. It has never been 

 possible, however, to eliminate wood entirely from the 

 superstructure, because metal responds so quickly to the 

 action of heat and cold that the unequal expansion and 

 contraction breaks the glass. Consequently, wood is 

 still used as a medium on which to place the glass. A 

 study of the relative strength of different species of wood 

 has made possible, however, the manufacture of wooden 

 sash-bars of much smaller dimensions. Improved methods 

 of manufacture of American glass have improved the qual- 

 ity greatly, and have made possible the use of larger sizes, 

 so that interior light efficiency has been greatly increased. 

 Foundations of wood have been replaced by those of con- 

 crete, and this has been an important step in increasing the 

 durability of houses. Foundations are especially impor- 

 tant in greenhouses, for as soon as they weaken, the super- 

 structure quickly gives way. Wooden interior supports 

 have given place to those of iron pipes, thereby eliminat- 

 ing much shade and increasing to a considerable degree, 

 the strength of the supports. 



