The House resolves itself into a Committee of the Whole to discuss appropriation bills, and sometimes for other legislation. The House of Commons also has several standing committees, each of which has responsibility for a particular area of government (for example, finance or transport). These committees oversee the relevant government departments, may hold hearings and collect evidence on governmental operations and review departmental spending plans. Standing committees may also consider and amend bills. Standing committees consist of between sixteen and eighteen members each, and elect their chairmen. Some bills are considered by legislative committees, each of which consists of up to fifteen members. The membership of each legislative committee roughly reflects the strength of the parties in the whole House. A legislative committee is appointed on an "ad hoc" basis to study and amend a specific bill. Also, the Chairman of a legislative committee is not elected by the members of the committee but is instead appointed by the Speaker, normally from among his deputies. Most bills, however, are referred to as standing committees rather than legislative committees. The House may also create "ad hoc" committees to study matters other than bills. Such committees are known as special committees. Each such body, like a legislative committee, may consist of no more than fifteen members. Other committees include joint committees, which include both members of the House of Commons and senators; such committees may hold hearings and oversee government, but do not revise legislation. Although legislation may be introduced in either House, most bills originate in the House of Commons. In conformity with the British model, the Lower House alone is authorized to originate bills imposing taxes or appropriating public funds. This restriction on the power of the Senate is not merely a matter of convention, but is explicitly stated in the "Constitution Act, 1867". Otherwise, the power of the two Houses of Parliament is theoretically equal; the approval of each is necessary for a bill's passage. In practice, however, the House of Commons is the dominant chamber of Parliament, with the Senate very rarely exercising its powers in a way that opposes the will of the democratically elected chamber. The last major bill defeated in the Senate came in 2010, when a bill passed by the Commons concerning climate change was rejected in the Upper House by a vote. A clause in the "Constitution Act, 1867" permits the Governor-General (with the approval of the Queen) to appoint up to eight extra senators to resolve a deadlock between the two houses. The clause was invoked only once, in 1990, when Prime Minister Brian Mulroney advised the appointment of an additional eight senators to secure the Upper House's approval for the Goods and Services Tax.