54 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



to destroy the blossoms of a whole houseful of Orchids. But they 

 have been successfully combated by covering in the house with 

 canvas sheets. Elsewhere I have seen this done, sometimes at 

 my suggestion, with beneficial results. Still, at the best, it is 

 but an expedient. Immunity obtained in this way is only 

 partial. Severe fogs of short duration, or longer ones of only- 

 moderate density, may be filtered through canvas, so that the- 

 damage caused is lessened ; but a persistent dense fog generally 

 prevails in the end. 



If plant-houses were constructed rather less leaky than is the 

 -case at present, something definite could no doubt be done to- 

 wards filtering the air. I confess to holding serious doubts as 

 to whether the admission of air to plant-houses, as in vogue just 

 now, is based on sound physiological principles — and this quite 

 apart from the fog-nuisance. During the course of my inquiries 

 into fog, a device for ventilating conservatories— the " patent fog- 

 annihilator" of Mr. Charles Toope — came prominently under my 

 notice ; and as I have been frequently asked what I think of it,. 

 I will take this opportunity of stating what I know. The system 

 is as follows : A number of boxes, situated on the floor under 

 the staging, communicate directly with the exterior by means 

 of apertures which can be readily closed if desirable. These 

 boxes contain several open-work trays, upon which sticks of 

 charcoal are loosely placed. The air entering a box from outsider 

 is led through these trays, coming into close contact with thechar- 

 coal. As the air leaves the box it impinges upon the hot-water 

 pipes, and is thus warmed before it reaches the plants in cultiva- 

 tion. The entrance of air is promoted by simple contrivances 

 known as ''exhaust-caps" placed on or near the ridge of the' 

 house. These caps are so constructed that practically, under all 

 conditions, an out-draught of air obtains. Should the draught 

 be too great, it can be regulated by means of valves. By this 

 system a constant circulation of air throughout the house is 

 brought about. The air enters the charcoal-box at once from 

 outside. It passes through this and is warmed by the hot-water 

 system of the house, and ultimately escapes by means of the 

 "exhaust-caps." Excepting for the apertures mentioned the 

 house is air-tight. It is by means of the charcoal that Mr- 

 Toope claims that the air admitted is purified. As the air cir- 

 culates between the sticks of charcoal it gives up the products 



