The question of the lighting of a gallery or room where pictures are to be displayed has been touched upon already at the beginning of the present chapter. We would now add that actual skylights are not without drawbacks. One of these, especially in the case of water-colours, is the presence in large proportion in the light from the zenith of those rays which act most energetically upon pigments, A few observations as to gas cannot be excluded. Gas, before and after burning, is bad for pictures. The evil effects of an occasional escape of unburnt gas are less to be dreaded than those caused by the products of gaseous combustion. These products are sulphuric acid, sulphurous acid, carbonic acid, and the moisture which is formed at the same time. Thence results a hot, moist atmosphere laden with these corrosive compounds. The water-vapour condenses into the liquid form and dissolves a part of the acids named above; the drops which trickle down are very injurious to paper, wood, canvas and pigments. In any case, all the products of the combustion of gas should be removed from the room as they are formed. For even when there may be no visible condensation of liquid, the vapours formed are often absorbed as such by paper, canvas, etc., and do in that form their destructive work.

An illustration of this fact is furnished by an analysis of the leather back of an old calf-bound volume. Owing to its absorption of the products of the burning of gas this back had decayed and fallen off, and was found to contain over 6 per cent. of free sulphuric acid.

With respect to the building itself in which pictures are to be kept, our aim should be to secure as far as possible pure air, an equable and agreeable but moderate temperature, and freedom from dust and dirt. Solidity of construction, a continuous damp-proof course, a certain degree of elevation above the ground-level, and double walls enclosing an air-space, are desirable as conducing to uniformity of temperature, and preventing the condensation of moisture upon the interior of the rooms. Due provision should be made for excluding from the galleries themselves the dust and dirt which may be brought in by visitors. And it cannot be too strongly urged that the supply of fresh air should not, as it were, accompany the visitors, but be brought in from an independent source. The place of in-take of such supply should not be gratings near to, or on the level of the ground, in out-of-the-way and dirty corners, the certain depositories of uncertain rubbish. From such sources air laden with organic and inorganic impurities can alone come. The question of the exclusion of fog and city-smoke may be mentioned here. Some kind of air-filter is useful.

It is astonishing how effectively the solid and liquid particles suspended in a yellow fog may be strained off and intercepted by passing the air through a layer of loosely packed carded cotton enclosed between two sheets of perforated zinc - this air-filter of course requires occasional renewal. Moist white lead, that is, white lead in powder reduced to a paste by admixture with water, will absorb the sulphuretted hydrogen as well as the sulphuric and sulphurous acid present in town air. And if the walls of the galleries are coated with a distemper paint containing white lead, this absorption of impurities goes on continually. For these impurities are more readily absorbed by an unprotected and properly-prepared distemper than by the pigments in the pictures. To secure this result the distemper should be made, not with size, but with starch-water, just sufficiently strong to bind the particles together and to the wall. For further particulars as to these and other arrangements for the conservation of pictures, especially in public galleries, the reader is referred to a paper on the subject in the Portfolio for 1882, pp. 106-108.

The conservation of pictures naturally leads us on to their restoration. Picture-restoration, like some other kinds of restoration, often involves injury, often renewal. It is frequently difficult, sometimes impossible, to reestablish the pristine state of the work. The operations involved should never be undertaken by the inexperienced amateur. And picture-restorers themselves are too often artists who have mistaken their profession, or who have been imperfectly trained. Many possess no power or appreciation of accurate draughtsmanship. Look, for illustrations, at those crucial parts, the hands and feet, in 'restored' pictures. Nor have they that exquisite sense of quality in colour and of delicate hues which will enable them to fill up properly actual gaps in a painted surface. Then the pigments they use are too often unsafe, and their vehicles unsatisfactory; so both soon alter, generally becoming darker and yellower. At the same time, the skill of some restorers in the matter of mechanical repairs - parquetting, transferring, relining, etc. - cannot be too highly extolled.

Several manuals of directions for restoring pictures have been given to the world; they are of very unequal value. Some of these tell you nothing, for they are intended merely to advertise the skill of the writers. Others advocate a treatment which may be called heroic, giving you solvents, not only for the discoloured varnish, but for glazings and paints. In reality, no directions can replace experience and skill. The late Max von Petten-kofer's method is one of the best known, but it is very rarely applicable with safety and success. The object of this method is the renewal of the transparency and continuity of the varnish by a process of re-solution in situ. With this intention, the picture is exposed in a closed shallow box to the action of the vapour evolved from moderately strong spirits of wine. This vapour dissolves the mastic on the surface of the picture, forming once more a spirit-mastic varnish. This, on exposure to the air, hardens, and leaves a shining coat of resin. But this resin, being necessarily discoloured and sinking into the cracks of the paint, makes them more prominent, while there is great danger of its being unequally distributed over the surface of the work.