In busy trading the shares topped the Footsie leader board, scoring a 43.5p gain to 1,172.5p; at one time they were 1,184p. In the stock market's setback last autumn they fell to 698p.
Rumours of a US link have often stirred BOC - once known as the British Oxygen Co - in the past few years.
But this time round there appears to be a growing conviction that a deal, perhaps even a bid from the American group, is likely to materialise.
The latest descent by Tyco of the US on Williams, the locks and security group, has helped to encourage thoughts of a Praxair deal. So have suggestions that sales in both the UK and European chemical industries are picking up. Higher gas prices in the US also helped sentiment.
BOC is expected to produce profits of around pounds 400m this year, which would compare with a depressed pounds 247.2m last time.
Continuing talk of corporate action helped Footsie achieve a modest 19.5 points gain to 6,431.5. At one time it was up 54.4. Supporting indices also edged ahead.
EMI, the showbiz group, again hit a high note on talk of a takeover strike with the shares hitting 480.25p, up 17p. Rupert Murdoch's News Corporation is said to be contemplating a 520p bid.
The controversial media tycoon's BSkyB, the satellite television station, rose 19.5p to 603p as French group Vivendi laid claim to a 17 per cent stake and signalled its hopes of doing a deal with the Murdoch empire.
Earlier this year BSkyB, where Murdoch interests have 40 per cent, held abortive talks with Canal Plus, the French TV group which is 34 per cent owned by Vivendi.
National Grid hardened 11.25p to 458.5p following suggestions of a French strike for Energis, its 49 per cent owned associate which put on 6p to 1,626p.
The scheduled Footsie changes had the predicted impact. Today Anglo American, the rather staid South African investment and mining group, is likely to be given clearance to join the exclusive share club, probably ousting computer group Sema.
Granada, which could accompany today's results with a share split, firmed 11p to 1,315p. Interim profits are expected to emerge 12 per cent higher at pounds 300m.
Insurer Prudential Corporation added 25p to 867.5p on talk that it plans to float its Egg banking off-shoot and BG was inspired by Credit Lyonnaise support and suggestions of a bid from Shell.
Vodafone's figures encouraged BT 23p higher to 1,122p and Colt Telecom 30p higher to 1,293p. Vodafone dialled a 16p gain to 1,298p, a peak.
Cadbury Schweppes, as its proposed deal with Coca-Cola hit further problems in Australia, retreated 10.5p to 435p.
British Steel gave up 4p to 146.25p on second thoughts about its Dutch deal, and some of the froth was blown off the drinks sector with Allied Domecq off 9.5p at 612.5p and Bass 25p off at 928.5p.
But Enterprise Inns moved to a new 473.5p high with a 6p gain. The pubs chain is said to be on the verge of a buy and sell deal with brewer Bass; it will take on 200 Bass pubs and sell some 20 of its existing collection to the brewer.
There is also talk of a renewed strike at Inn Business, little changed at 76.5p, which is in talks with a venture capitalist.
De La Rue, the security printer, shaded to 350.5p ahead of an Investec Henderson Crosthwaite investment dinner at London's Claridges Hotel. Around 20 fund managers attended.
Computacentre, which has flopped since its debut last year, logged a 40p gain to 430p, a far cry from the 783p peak achieved just after the flotation.
Viglen Technology, the latest to strike a pact with US giant Microsoft, surfed 8p higher to 40.5p. The company headed by Alan Sugar is to develop an information technology training network.
VDC, the animal healthcare company, firmed 10p to 235p as Genus, an Ofex- traded cattle breeding group, won a protracted and acrimonious bid battle with a third offer - 235p a share.
Lawrence, an animal group which had fixed up a defensive merger with VDC, fell 10p to 395p. On Ofex, Genus gained 15p to 160p.
Arjo Wiggins Appleton, the packaging and paper group, improved 5.5p to 200.5p - an analysts' meeting is due to be held today.
AromaScan, the electronic nose group, was also helped by a pending investment meeting, scheduled for later this month, adding 1.25p to 18.75p.
New Look, the clothes retailer, fell 4.5p to 199p - analysts meet the chain next week.
Spring Ram, the battered bathrooms group, jumped 2.5p to 13.75p on talk of a major disposal which will leave the group with little more than a cash shell.
Cedric Brown, the controversial ex-British Gas executive, has at last moved in as chairman of Atlantic Caspian, the oil explorer, fuelling the price just 2.3 per cent higher to 11p.
KBC Advanced Technologies was hit by another profits warning, crashing 40.5p to 117.5p. Williams de Broe said sell.
Fastrack was another unsettled by trading caution, falling 1.5p to 10p. AB Airlines lost even more altitude as the market fretted about prospects - the shares fell 3.5p to a 16p low.
Gowrings, the garages and fast-food group, which later this month will switch to the breweries and pubs sector, reversed 9.5p to 132p.
SEAQ VOLUME: 978.7
SEAQ TRADES: 71.241
GILTS INDEX: 106.66 -0.35
THE SMALL but highly ambitious timber merchant John Mansfield is gathering strength, prompting thoughts that a deal is being prepared.
The shares in the past few weeks have climbed from 4.25p to close at 7.75p, up 0.25p. The company is the vehicle for the South African Myerson and Treger duo and has had Stuart Wallis as chairman. Last year it made an unsuccessful bid for the Norcros building materials group.
CHORION, being revamped by former First Leisure chief executive John Conlon, is nudging its 12-month high at 27.5p.
Stock broker Kyte Securities believes the shares should hit 35p in the next six months.
The group's London bars and nightclubs have "exciting growth potential" and the SegaWorld venture at London's Trocadero centre is likely to be unwound next year, reducing a drag on profits.Reuse content