The Seattle Star., December 18, 1914 - England Guards Secret Engine of Destruction Which Could Wipe Germans Off Map in Hour
The Tacoma Times. December 14 1914 - UK has WMD that would kill 1/3 of the population of Europe, “Dundonald’s destroyer”
Short answer:
Its poison gas, but it doesn’t appear that the author of the news story knows that. Ironically it’s likely that the Seattle newspapers were writing a second hand story based on rumors - that was by accident closer to accuracy than they had any way of knowing.
Discussion:
Admiral Lord Cochrane, 10th Earl of Dundonald
"Dundonald" is better known to us as the Admiral Lord Cochrane [1775-1860] who was an aggressive and innovative British naval officer, who also had a notable career in Latin America. He's also a bit of the inspiration for the character of "Jack" Aubrey in Patrick O'Brien's celebrated naval historical fiction
Cochrane always had a scheme or three underway -- remarkable in a service as conservative as the Royal Navy (at least prior to Jacky Fisher)-- and many of these were technically innovative, from a tunneling machine, to steamships to the idea of "stink ships", vessels filled with layers of coal and sulphur which could be set afire and towed or sailed into an enemy port-- one of the first modern notions of a poison gas attack. Cochrane apparently gets this idea in 1811, when his ship stops in Sicily, and he sees sulphur being mined (and burned)-- he proposes it to the Prince Regent and the Admiralty in 1812, and while its rejected, he continues to push for it even decades later . . . he resubmits the idea in the 1840s and 1850s, the physicist chemist Michael Faraday is given the task of investigating Cochrane's proposal and is skeptical; it goes nowhere.
Cochrane's design was little more than a noxious pipedream, finds its way into the public imagination. Late in the century, for example, New Zealand was possessed by a panic that Russian vessels were lurking offshore, ready to pump something called "water gas".
You can see why this would make great copy -- naval technology had advanced very rapidly, and the notion of enemy ships lurking just over the horizon, ready to poison a sleeping city, that was a potent fear. One might even detect a hint of the fear of contagion that was familiar in port cities; this was a time when public health authorities were very concerned about plague or other diseases arriving by sea.
In addition to Cochrane, there are other British scientists who advocate things along these lines, notably the chemist Lyon Playfair-- his memoirs become public at the end of the 19th century. This was a time of tinkerers and inventors, and there's not a small number of 19th century inventors touting some form of poison gas-- but the use of the name "Dundonald" in your sources makes clear that they're thinking of Cochrane, even though he'd been dead for half a century at this point.
Douglas Cochrane, 12th Earl Dundonald [1852-1935]
The reason for this later interest in the “Secret War Plans” likely proceeds from the activities of Cochrane’s grandson, the 12th Earl of Dundonald. Like his grandfather he had a fascination with military technology, though he's an Army man, serving in Africa (Sudan, Boer War) and ending his career at a Lieutenant General. He innovates "Dundonald's galloping carriage", a sort of mobile machine gun-- a Maxim gun mounted to a wagon platform. But in 1914, he's promoting the "secret weapon" whose plans the family have kept as a kind of national treasure, albeit one in which the nation has previously had no interest.
What kind of public awareness is in there in 1914 of the possibilities of gas warfare? Some. As noted above, there had been panics over unspecified "gas" attacks in earlier decades. The Japanese use gas at Port Arthur in 1904, to some effect. That Dundonald's "secret weapon" was an asphyxiating gas-- is there any public awareness of that? We can find newspaper references to rumors about it many years earlier-- for example in 1901, the Manchester Courier and Lancashire General Advertiser reports on a speech given by Arthur Balfour to the Primrose League, where he states:
I have heard a story - I do not know whether it is true- of that great sea captain, Lord Dundonald that he invented a kind of chemical weapon which produced fumes so abominable and destructive that, if they were let loose over some beleaguered city, it would be impossible for that city to hold out.
Balfour is the Foreign Minister at the time, and well acquainted with the nastier side of war - but he's plainly not taking 'Dundonald's Secret Weapon" seriously; in this speech he uses it as a metaphor for a bad idea, not a practical possibility.
Where do the articles cited in the question come from?
So here's the curious thing-- your sources are December 1914, and in the fall of 1914 the 12th Earl has meetings to promote the idea with Kitchener (no interest) and Churchill (interested). These plans really were treated as "secret" by the Dundonalds, to be revealed to the authorities at a time of "national emergency"; with the outbreak of WW I, this is what the 12th Earl does. Later in 1915, Dundonald becomes "Chairman of the Admiralty Committee on Smoke Screens" -- but their work is essentially unconnected to the actual use of poison gas in the war.
So one reading of these news stories is that they're coming from Dundonald himself or Churchill as a way to light a fire under a military that has other priorities. The other possibility is that in the frenzy of war fever, the press is on its own digging up old stories of secret weapons. Searching through 1914 newspaper stories, in November 1914 we have an article "Dundonald's Destroyer" that appears in The North American Review; this is likely the source for the Seattle articles, and the author of the former article plainly doesn't know what the weapon is:
In brief, we know pretty certainly what it was not; and must conclude that it was some mechanical, electrical, or chemical device such as never has been hit upon by another and thus has never been put into practice, or else that Dundonald himself and half a dozen of the most accomplished scientific and military experts of his time were victims of a most extraordinary delusion
So one plausible reading of this story is that the newspapers were ginning up old rumors into a sensationalist story -- and accidentally were much closer to the truth than they had any way of knowing!
Sources: