JlNE, 1901. ^ 



KNOWLEDGE. 



137 



By the application of certain processes the surface of 

 standard silver may be made to assume various tints. 

 by which means the ai'tistic effect is increased. 



When silver-copper alloys are heated to dull redness, 

 with access of air, the surface becomes blackened, owing 

 to the formation of a film of copper oxide, but by 

 blanching — that is, immersion in hot dilute sulphuric 

 acid — the copper oxide is dissolved, and the silver 

 assumes a beautiful dead-white appearance known as 

 ■' frosted silver. 



When silver articles have been thus frosted and the 

 portions in relief are burnished, very effective results 

 are often obtained, and the beauty of the ware greatly 

 enhanced. 



Articles of plate maj' be '' frosted,' oi-, as it is some- 

 times termed. " matted or deadened," also by boiling 

 them in bisulphate of potash, which acts in the same 

 way as dilute sulphui'ic acid. 



In order to obtain on ornamental silver wares the effect 

 known as oxidized silver, the surface is darkened by 

 a thin film of silver sulphide, produced by boiling 

 the articles in an aqueous solution of potassium sulphide, 

 and then i-ubbing with very fine pumice, which re- 

 moves the dark layer of silver sulphide from the portions 

 in high relief and leaves dark lines in the more deeply- 

 cut recesses. 



The effect thus produced is frequently very sti'iking, 

 and adds materially to the appearance of the article. 



At the Paris and London Mints** silver medals have 

 recently been subjected to the process known as " sand 

 blasting," by the aid of an appliance which projects 

 against the surface of the medal a small jet of air carry- 

 ing with it fine sand, and having a velocity of about 

 180 feet per second. When thus treated, the surface 

 of the medal becomes " frosted," and may be subse- 

 quently oxidized as above described, or the following 

 process may be adopted. The " frosted " medal is 

 immei'sed in an alcoholic solution of platinum chloride, 

 whereby the surface becomes blackened by the deposition 

 of a thin film of metallic platinum. Subsequent rubbing 

 with a brush and very fine pumice changes the blackened 

 surface to a delicate gi^ey, and if this operation is con- 

 ducted skilfully, graduated shadows may be left where- 

 ever the artist considers their presence to be desirable. 

 The beauty of the medals so treated, and the fidelity 

 with which the details of the design are revealed, is 

 beyond question, but it may be doubted whether the 

 surface of the medal is permanently protected. 

 {To be concliithfl.) 



TO 



THE WHITE NILE-FROM KHARTOUM 

 KAWA. 



AN ORNITHOLOGIST'S EXPERIENCES IN THE SOUDAN. 



By Harry F. Witiierby, f.z.s., m.b.o.u. 



II.— THE RIVER— E.SSEXTIAL ALIKE TO MAN, 

 BEAST AND BIRD. 



Having obtained all the necessary permits as well as bag- 

 gage and riding animals, oiir next task wa.s to engage 

 ser\'ants. We eventually gathered together a motley 

 and somewhat amusing crowd. The chief of these — one 

 Hassan Mahomet Eshari, an Egyptian — we brought 

 from Cairo, and he proved in every way an excellent 

 interpreter and hard-working headman and servant. 

 Mustapha, who hailed" from Berber, was a cook as well 



»• 28th Report of Riyal Mint, 1897, \k J7. 



as a lazy and perfidious rascal. Mahomet— an old 

 kingly-looking Turk, whose ankles were callous through 

 the shackles of the Khalifa — was " sais " to the donkeys, 

 lie was a fine figure, and generally rode in front of our 

 caravan, ordering the natives out of the path, or com- 

 manding them to come and show the way for a mile 

 or two, all in a most dignified manner. But when it 

 came to work in camp his one idea was to sit on his 

 haunches and smoke, and to obtain the services of the 

 nearest wandering boy to wait upon him and his donkeys. 

 The natives feared him much, and generally obeyed him 

 without a murnuir, but all our hard sayings and harder 

 doings failed to make much permanent improvement 

 in his habits. 



Wc had great expectations of Mirsal, an old Soudanese 

 soldier, who was to act as a gun-bearer. When 1 

 engaged him he went through a most charming panto- 

 mime. Gripping his right arm with his left hand, he 

 swore by the Prophet's beai'd that so long as there was 

 strength in that arm ho was my most faithful servant. 

 A few days later, when on the march, we sent him over 

 to a village on the other side of the river with money 

 to buy food and tobacco. We never saw him again, 

 nor could policemen mounted on camels find him. But 

 maybe wc misjudged him. The strength may have for- 

 saken his arm. Our camel men were from Dongola. 

 They were a lively, irresponsible crew, requiring much 

 physical persuasion. On the whole, we were not greatly 

 impressed by the reliability of the natives of the Soudan, 

 as far as concerns either work or honesty. 



Our final plan was to travel up the river quickly, 

 and to return slowly. While marching up the river 

 we, of course, should have no time for collecting or 



Muliomet at Work. 



Ft-oin (I pholo'ji-*tiih l_i>i Mr. »_', F. CambukN. 



preserving, but we should be able to make notes of the 

 birds and the country, and thus obtain a knowledge 

 of the ground which woxdd greatly assist us on our 

 return in deciding where to make collecting camps, and 

 how long to stay at each. 



With much kind help from many in authority at 

 Omdurman all our preparations were at last complete, 

 and on the afternoon of March 20th, we sent our men, 

 animals, and baggage across the White Nile to the 

 Khartoum, or east, bank. Joining them early the next 

 morning wc found to our gieat relief that there were no 

 deserters, and, moreover, that all disputes as to which 

 box or package should be carried on which camel were 

 settled, and all were ready to start. The apportion- 

 ment of loads and the loading up at the start arc always 

 difficulties, and never before had we experienced so little 

 trouble. 



