150 



KNOWLEDGE. 



April, 1913. 



creatures he has read of at home. It was recently my luck to 

 explore some of the little-frequented parts of the Parana, four 

 hundred miles up the river. Among the many 

 interesting objects were the famous Oven Birds, 

 of which there are various species. The com- 

 monest {Furnarius rufus) is a bird some- 

 what smaller than a Thrush, of dingy plumage 

 and with a continuous cry. The nests (see 

 Figures 146-148) are placed in the most 

 conspicuous positions — on the cross bars of 

 telegraph poles, window ledges, on monuments 

 in cemeteries, and on posts by the roadside, 

 and I have seen one built in a small back 

 yard on a post to which a clothes line was 

 attached. The clothes fluttering, the woman 

 passing to and fro, the children and dogs 

 playing around caused no uneasiness on the 

 part of the birds. No one ever seemed to 

 resent the intrusion of the birds in apparently 

 inconvenient spots — perhaps the charac- 

 teristic " slackness " of the people accounts 

 for the fact that the abandoned nests are 

 allowed to remain until totally disintegrated 

 by the weather. 



The name Oven Bird is singularly appro- 

 priate, the clay nests bearing a striking 

 resemblance to the Spanish ovens in common 

 use throughout the Argentine. The natives 

 call the bird hornero (baker), or sometimes 

 casern, which seems to mean housekeeper. 

 There are two types of these nests ; one 

 kind having a large circular entrance (see 

 Figure 148), the other with the entrance 

 obstructed by a sort of half-opened partition 

 (see Figure 147). The nests are about a 

 foot high and an inch in thickness, and 

 weigh nine or ten pounds. 



Lionel E. Adams. 



Figure 147. 



A Nest of the Oven Bird 

 with a valve-door. 



PHOTOGRAPHY. 



By Edgar Senior. 



FADING OF SILVER PRINTS.— 

 Having had occasion recently to look over a 

 number of prints made by var- 

 ious processes some sixteen 

 years ago, and in which every 

 care was taken at the time to 

 make as lasting as possible, 

 such was the condition in 

 which nearly all of those in 

 which silver or its compounds 

 formed the image was found, 

 that we were forcibly reminded 

 of the fact that carbon and 

 platinum are the only really 

 reliable processes for per- 

 manency that can be depended 

 upon. It is only fair, how- 

 ever, to mention that those 

 of the prints which were made 

 upon collodio - chloride of 

 silver paper had withstood the 

 test of time and atmospheric 

 influences in a most perfect 

 manner, exhibiting no change 

 whatever that was perceptible. 

 As this want of permanency 

 in prints may be, and often is, 

 a serious matter, one is necess- 

 arily led to the consideration 

 as to what are the conditions 

 under which silver prints are 



liable to fade. It has been stated, and no doubt with a good 

 amount of truth, that the combined bath method is responsible 



Figure 148. 



A boy holding the Nest seen in 



Figure 146 ; note the open 



circular entrance. 



Figure 149. 

 A Spanish Mud Oven at Colastine 



for a considerable amount of the trouble, as these baths usually 

 contain lead salts, added for the purpose of making the toning 

 more regular, and as no amount of washing 

 appears to entirely remove them, the ultimate 

 result is, that the whites of the print become 

 discoloured from the formation of lead 

 sulphide. Without in any way wishing to 

 uphold this method with the combined bath, 

 we must say that prints made upon Lumiere's 

 citrate of silver paper, and toned in such a 

 bath, remain unaltered after the lapse of 

 some years, not that this can be taken as a 

 proof of permanency. The question is : 

 are prints made by any particular method 

 liable to fade, and to what extent are they 

 likely to do so ? As a test of permanency it 

 has been recommended to photographers to 

 expose a print " in a moist atmosphere " 

 to sunlight for several days when absence 

 of change in colour was to be considered as 

 proof of permanency. This test, at most, is 

 very crude and incomplete, as prints which 

 have withstood it have changed under other 

 conditions. Then, again, many papers will 

 change in colour if exposed to sunlight for 

 a few days, some assuming a yellow, others 

 a greenish tint, and this alteration in the 

 colour must not be mistaken for fading of 

 the print. This latter is brought about by a 

 change in the silver image as the result of 

 chemical action upon it, and must be studied 

 from the nature of the silver print itself. It is 

 well known that although gold, and especially 

 platinum, remains untarnished by any atmos- 

 pheric conditions, silver, on the other hand, 

 readily becomes coated with a yellowish filth 

 of sulphide, due to the presence of sulphuretted 

 hydrogen or more complex compounds of 

 sulphur in the atmosphere, from the products 

 of combustion of gas, the burning of coal, 

 and the decomposition or decaying of organic 

 matter in the presence of sulphates, the action 

 of the hydrogen sulphide upon silver prints 

 in all probability being accelerated by the 

 presence of moisture. Other conditions which 

 would also favour chemical 

 action are increase in temper- 

 ature, and the state of division 

 of the bodies reacting. And 

 when the photographic image 

 is made up of silver in a fine 

 state of division, we can under- 

 stand why it is less able to 

 resist the action of such 

 bodies as sulphuretted hydro- 

 gen. Then, again, there are 

 many substances which, 

 though having the same 

 composition chemically, exist 

 under several modifications 

 having entirely different pro- 

 perties. Silver is one of these, 

 and this may account for a 

 certain kind of silver image 

 being more permanent than 

 another. In the case of a 

 silver print upon albumen, 

 gelatine, or collodio-chloride 

 paper, the image is probably 

 little more than a darkened 

 organic silver compound, while 

 that on bromide paper consists 

 of metallic silver. This may 

 be seen by examination under 

 a microscope, when the particles of silver will be readily 

 observable, whereas under the same power a printed-out 



