604 Professor A, H. Church [April 12, 



generally made. The cost of the material was quite trivial, the 

 expense of the treatment being practically confined to the cost of 

 labour and scaffolding. Let me add that the cheapest and best 

 source of the baryta is the fused crystalline hydrate, which contains 

 about 58 per cent, of oxide, is less liable to carbonation than the 

 ordinary crystals, and corresponds to the formula BaHaO.,, 5 HoO. 



With regard to the treatment of the decayed limestone of 

 Canterbury Cathedral, and especially of Bell Harry Tower, a few 

 words only are required. Between November 1001 and June 1902, 

 I made a series of analyses of decayed stones from this tower, and 

 recommended the architect in charge to adopt the treatment with 

 baryta, as an experiment on a small surface. This was to have been 

 done in the summer of 1002, but I heard nothing further of the 

 matter until more than two years afterwards, when I learned that 

 under other advice, the discredited process of the late Jesse Rust, 

 described in his incompleted Patent of June 6, 1861, alternate 

 washes of baryta and fluosilicic acid, but modified by using the 

 fluosilicic acid in smaller proportion than he recommended, was to 

 be tried alongside of the simple baryta solution. I have always 

 deprecated the application of a free acid, such as fluosilicic, to stone- 

 work injured by sulphuric acid. For it liberates carbon dioxide 

 from any carbonates present, and at least sometimes produces an 

 unsightly white efflorescence. My own patent process of 1862, 

 according to which alternate applications of baryta-water and of a 

 dialysed solution of silica were to be made, causes no efflorescence 

 and no liberation of CO.^, and has been used with success on large 

 surfaces of new stone. But the silica clogs the pores of the stone, 

 and, if employed at all, should be reserved for the final dressing. 

 My own convictions as to the best course to be pursued at Canter- 

 bury, as expressed in my letter to The Times of November 10, 1904, 

 remain unaltered in the main, although there are cases in which a 

 hydrofuge substance, such as paraffin or ceresin, may be safely used. 

 But I have never found baryta to fail, where its employment was 

 indicated, on calcareous stone. Nor does disruption ever occur with 

 the repeated but reasonable application of baryta-water saturated 

 at 16^ C, although I can easily believe that warm and stronger 

 solutions might be dangerous to use. 



Last year five bays of the Cheapside frontage of Mercers' Hall 

 were treated with baryta. The stone was suffering from serious 

 decay, so that some loose portions had to be removed. The treat- 

 ment with baryta was followed, in the case of the three easternmost 

 sections of the frontage, by another treatment intended to render 

 the re-consolidated stone acid-proof. I had used the second method, 

 with apparent success, in protecting mural paintings in West- 

 minster Palace. It consists in applying to the surface a paste or 

 ointment of ceresin, a solid paraffin wax from ozokerite. The sample 

 used had a solidifying point of 156° F. ; it was made into a paste 



